Correct the transmit protocol for Nordic TWI. (#417)

* Correct the transmit protocol for Nordic TWI.
The register address and the data must be sent in one operation, as
specified in the nRF51 Series Reference Manual version 3.0.

  * arch/ARM/Nordic/drivers/nrf_common/nrf-twi.adb:
    (Master_Transmit): Clear any previous Suspend/Stop settings before
      the transfer.
      Only do a Stop_Sequence if requested.
    (Mem_Write): Call Master_Transmit to send the data as well as the
      register address in one transaction. This requires moving the
      Do_Stop_Sequence setting to before this single transaction.

* Revert Nordic TWI Mem_Write to original.

Avoids creating a potentially large data buffer on the stack.

  * arch/ARM/Nordic/drivers/nrf_common/nrf-twi.adb (Mem_Write): reverted to
      sending the register address without a stop sequence, followed by the
      data, followed by a stop sequence.

Author: simonjwright

Diff for: arch/ARM/Nordic/drivers/nrf_common/nrf-twi.adb

@@ -1,6 +1,6 @@
 ------------------------------------------------------------------------------
 --                                                                          --
---                    Copyright (C) 2016-2020, AdaCore                      --
+--                    Copyright (C) 2016-2022, AdaCore                      --
 --                                                                          --
 --  Redistribution and use in source and binary forms, with or without      --
 --  modification, are permitted provided that the following conditions are  --
@@ -134,6 +134,11 @@ package body nRF.TWI is
       --  Set Address
       This.Periph.ADDRESS.ADDRESS := UInt7 (Addr / 2);
 
+      --  Configure SHORTS to neither suspend nor stop the TWI port
+      --  after the next byte transfer.
+      This.Periph.SHORTS.BB_SUSPEND := Disabled;
+      This.Periph.SHORTS.BB_STOP := Disabled;
+
       --  Prepare first byte
       This.Periph.TXD.TXD := Data (Data'First);
 
@@ -201,7 +206,6 @@ package body nRF.TWI is
       --  Set Address
       This.Periph.ADDRESS.ADDRESS := UInt7 (Addr / 2);
 
-
       if Data'Length = 1 then
          --  Only one byte to receive so we stop at the next one
          This.Periph.SHORTS.BB_STOP := Enabled;