serial: pch: simplify pop_tx() even more

1) take uart_tx_stopped into account every loop (the same as other uart
   drivers)
2) no need for 'count' variable, operate on 'size' directly

This allows inlining this into handle_tx() nicely in the next patch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220503080808.28332-5-jslaby@suse.cz


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