media: cx24116: prevent overflows on SNR calculus

as reported by Coverity, if reading SNR registers fail, a negative
number will be returned, causing an underflow when reading SNR
registers.

Prevent that.

Fixes: 8953db79 ("V4L/DVB (9178): cx24116: Add module parameter to return SNR as ESNO.")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>