V4L/DVB (13380): sms-cards: make id unsigned in sms_get_board()

Make id signed so we can't get an invalid pointer when we pass a negative
id.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>