[media] hdpvr: hide unused variable

The i2c client pointer is only used when CONFIG_I2C is set, and
otherwise produces a compile-time warning:

drivers/media/usb/hdpvr/hdpvr-core.c: In function 'hdpvr_probe':
drivers/media/usb/hdpvr/hdpvr-core.c:276:21: error: unused variable 'client' [-Werror=unused-variable]

This uses the same #ifdef to hide the variable when the code using
it is hidden.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>