media: bttv-input: better handle errors at I2C transfer

The error handling logic at get_key_pv951() is a little bit
akward, with produces this false positive warning:

	drivers/media/pci/bt8xx/bttv-input.c:344 get_key_pv951() error: uninitialized symbol 'b'.

Do a cleanup. As a side effect, it also improves its coding
style.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>