V4L/DVB (6148): Fix a warning at saa7191_probe

saa7191.c: In function 'saa7191_probe':
saa7191.c:596: warning: passing argument 3 of
	'saa7191_write_block' discards qualifiers from pointer target type

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>