tty: serial: use FIELD_GET() in lqasc_tx_ready()

FIELD_GET() can do the job smarter and more readable. We don't even need
ASCFSTAT_TXFREEOFF. So switch to the former and remove the latter.

Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220927111819.18516-2-jslaby@suse.cz


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