serial: core: Extract uart_alloc_xmit_buf() and uart_free_xmit_buf()

After conversion to the kfifo, it becomes possible to extract two helper
functions for better maintenance and code deduplication. Do it here.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240409174057.1104262-1-andriy.shevchenko@linux.intel.com


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