media: cxusb, dib0700: ignore XC2028_I2C_FLUSH

The XC2028_I2C_FLUSH only needs to be implemented on a few
devices. Others can safely ignore it.

That prevents filling the dmesg with lots of messages like:

	dib0700: stk7700ph_xc3028_callback: unknown command 2, arg 0

Cc: stable@vger.kernel.org
Fixes: 4d37ece7 ("[media] tuner/xc2028: Add I2C flush callback")
Reported-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>