serial: pch: remove xmit circ_buf size double check

One is in handle_tx() (as "min(xmit->head - xmit->tail, fifo_size))",
another one in pop_tx() (as uart_circ_empty(xmit)). So keep only the
latter.

This makes the code simpler and size variable is not needed now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220503080808.28332-4-jslaby@suse.cz


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