drivers/gpio/da8xx_gpio.c: Fix build warning

Fix:
da8xx_gpio.c: In function 'gpio_toggle_value':
da8xx_gpio.c:208:23: warning: variable 'bank' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>