tty: 8250: Add note for PX-835

The PX-835 and PX-846 are variants of each other, clarify that
in the comment for the card.
This patch makes no functional difference.

Signed-off-by: Cameron Williams <cang1@live.co.uk>
Link: https://lore.kernel.org/r/DU0PR02MB78998840C2EBED91AABD4494C4DBA@DU0PR02MB7899.eurprd02.prod.outlook.com


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