[nrf fromtree] dp: swdp_bitbang: fix unused variable build error

The variable config in sw_port_off() is not used, and it's
causing CI build error about unused variable. So remove it.

Signed-off-by: Daniel Leung <[email protected]>
(cherry picked from commit 1ec5ce0)

Author: dcpleung

drivers/dp/swdp_bitbang.c

@@ -677,7 +677,6 @@ static int sw_port_off(const struct device *dev)
 
 static int sw_gpio_init(const struct device *dev)
 {
-	const struct sw_config *config = dev->config;
 	struct sw_cfg_data *sw_data = dev->data;
 	int ret;