media: tvp5150: get rid of some warnings

media: tvp5150: get rid of some warnings

The recent tvp5150 patchset added two new warnings:

drivers/media/i2c/tvp5150.c: In function 'tvp5150_querystd':
drivers/media/i2c/tvp5150.c:829:18: warning: unused variable 'decoder' [-Wunused-variable]
  struct tvp5150 *decoder = to_tvp5150(sd);
                  ^~~~~~~
drivers/media/i2c/tvp5150.c:1522:6: warning: no previous prototype for 'tvp5150_volatile_reg' [-Wmissing-prototypes]
 bool tvp5150_volatile_reg(struct device *dev, unsigned int reg)
      ^~~~~~~~~~~~~~~~~~~~

Get rid of them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>