V4L/DVB (5215): Experimental support for signal strength/BER/uncorrectable count

After studying many hours worth of register dumps of MT352 and ZL10353 fed
with identically damaged RF signals I have made an educated guess at which
registers contain the AGC level, bit error rate and uncorrectable error
count values.
Implement the IOCTLs that return these values to userspace.

Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>