serial: 8250_omap: Convert to use uart_xmit_advance()

uart_xmit_advance() provides a common way on how to advance
the Tx queue. Use it for the sake of unification and robustness.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220909091258.68886-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>