serial: Convert SERIAL_XMIT_SIZE to UART_XMIT_SIZE

Both UART_XMIT_SIZE and SERIAL_XMIT_SIZE are defined. Make them all
UART_XMIT_SIZE.

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220624205424.12686-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>