serial: uartlite: Fix compilation warnings

Do not setup userial_ports array as const because
in uartlite_serial_putc is out_be32 which can't write
to tx_fifo if is const.

Warning log:
serial_xuartlite.c: In function 'uartlite_serial_putc':
serial_xuartlite.c:60: warning: initialization discards qualifiers from pointer target type
serial_xuartlite.c: In function 'uartlite_serial_getc':
serial_xuartlite.c:78: warning: initialization discards qualifiers from pointer target type
serial_xuartlite.c: In function 'uartlite_serial_tstc':
serial_xuartlite.c:87: warning: initialization discards qualifiers from pointer target type

Signed-off-by: Michal Simek <monstr@monstr.eu>