tty: 8250: Fix port count of PX-257

The port count of the PX-257 Rev3 is actually 2, not 4.

Fixes: ef5a03a2

 ("tty: 8250: Add support for Brainboxes PX cards.")
Cc: stable@vger.kernel.org
Signed-off-by: Cameron Williams <cang1@live.co.uk>
Link: https://lore.kernel.org/r/DU0PR02MB7899C804D9F04E727B5A0E8FC4DBA@DU0PR02MB7899.eurprd02.prod.outlook.com


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