[media] tuner-xc2028: Don't read status if device is powered down

That removes those timeout errors:

[ 3675.930940] xc2028 19-0061: Device is Xceive 3028 version 1.0, firmware version 2.7
[ 3676.060487] xc2028 19-0061: divisor= 00 00 8d d0 (freq=567.250)
[ 3676.349449] xc2028 19-0061: Putting xc2028/3028 into poweroff mode.
[ 3698.247645] xc2028 19-0061: xc2028_get_reg 0002 called
[ 3698.253276] em2860 #0: I2C transfer timeout on writing to addr 0xc2
[ 3698.253301] xc2028 19-0061: i2c input error: rc = -121 (should be 2)
[ 3698.253327] xc2028 19-0061: xc2028_signal called
[ 3698.253339] xc2028 19-0061: xc2028_get_reg 0002 called
[ 3698.259283] em2860 #0: I2C transfer timeout on writing to addr 0xc2
[ 3698.259312] xc2028 19-0061: i2c input error: rc = -121 (should be 2)

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>