media: em28xx-input: improve error handling code

The current I2C error handling logic makes static analyzers
confused:

	drivers/media/usb/em28xx/em28xx-input.c:96 em28xx_get_key_terratec() error: uninitialized symbol 'b'.

Change it to match the coding style we're using elsewhere.

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