tty: serial: extract lqasc_tx_ready() from lqasc_tx_chars()

tty: serial: extract lqasc_tx_ready() from lqasc_tx_chars()

The condition in lqasc_tx_chars()'s loop is barely readable. Extract it
to a separate function. This will make the cleanup in the next patches
easier too.

(Put it before lqasc_start_tx(), so that we can use it there later.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-5-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>