V4L/DVB: xc5000: Fix a few warnings

drivers/media/common/tuners/xc5000.c: In function ‘xc_write_reg’:
drivers/media/common/tuners/xc5000.c:298: warning: passing argument 3 of ‘xc5000_readreg’ from incompatible pointer type
drivers/media/common/tuners/xc5000.c:235: note: expected ‘u16 *’ but argument is of type ‘u8 *’
drivers/media/common/tuners/xc5000.c: At top level:
drivers/media/common/tuners/xc5000.c:223: warning: ‘xc_read_i2c_data’ defined but not used

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