[SERIAL] serial_cs: Use clean up multiport card detection

- Use ARRAY_SIZE() instead of home grown based version.
- use parse->manfid.card rather than le16_to_cpu(buf[1]) -
  manfid.card is already converted to this format.
- use info->prodid in subsequent tests rather than
  parse->manfid.card.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>