drivers: serial: uart_max32: Fixing bugs and code optimizations

- Defined 'api_configure' function outside of
'CONFIG_UART_USE_RUNTIME_CONFIGURE' macro because it is the only
configuration function and used in device initialization function.
- Removed unnecessary 'IF_ENABLED' part.

Signed-off-by: Furkan Akkiz <hasanfurkan.akkiz@analog.com>