[media] mb86a20s: apply mask to val after checking for read failure

Appling the mask 0x0f to the immediate return of the call to
mb86a20s_readreg will always result in a positive value, meaning that the
check of ret < 0 will never work.  Instead, check for a -ve return value
first, and then mask val with 0x0f.

Kudos to Mauro Carvalho Chehab for spotting the mistake in my original fix.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>