[media] hd29l2: fix sparse error and warnings

drivers/media/dvb-frontends/hd29l2.c:29:18: warning: Variable length array is used.
drivers/media/dvb-frontends/hd29l2.c:34:32: error: cannot size expression
drivers/media/dvb-frontends/hd29l2.c:125:5: warning: symbol 'hd29l2_rd_reg_mask' was not declared. Should it be static?

Variable length arrays are frowned upon, so replace with a fixed length and check
that there won't be a buffer overrun.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>