V4L/DVB (13687): cx231xx: use NULL when pointer is needed

The gpio field in the cx231xx_board.input structure is a pointer. Eliminate the
following sparse warnings (see "make C=1"):
 * cx231xx-cards.c:72:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:77:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:84:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:111:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:116:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:123:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:151:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:156:13: warning: Using plain integer as NULL pointer
 * cx231xx-cards.c:163:13: warning: Using plain integer as NULL pointer

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>