tty: serial: extract tx_ready() from __serial_lpc32xx_tx()

tty: serial: extract tx_ready() from __serial_lpc32xx_tx()

The condition in __serial_lpc32xx_tx()'s loop is barely readable.
Extract it to a separate function. This will make the cleanup in the
next patches easier too.

Cc: Vladimir Zapolskiy <vz@mleia.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-6-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>