serial: uart_sam0: Don't use CONFIG_ namespace for non Kconfig
serial: uart_sam0: Don't use CONFIG_ namespace for non Kconfig

The driver was defining a macro with a CONFIG_ prefix but this wasn't
coming from Kconfig.  Change the macro name not to conflict with the
Kconfig CONFIG_ namespace.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

explora26/zephyr,ldts/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr