i2c: cbus-gpio: set atomic transfer callback

CBUS transfers have always been atomic, but after commit 63b96983
("i2c: core: introduce callbacks for atomic transfers") we started to see
warnings during e.g. poweroff as the atomic callback is not explicitly set.
Fix that.

Fixes the following WARNING seen during Nokia N810 power down:

[  786.570617] reboot: Power down
[  786.573913] ------------[ cut here ]------------
[  786.578826] WARNING: CPU: 0 PID: 672 at drivers/i2c/i2c-core.h:40 i2c_smbus_xfer+0x100/0x110
[  786.587799] No atomic I2C transfer handler for 'i2c-2'

Fixes: 63b96983

 ("i2c: core: introduce callbacks for atomic transfers")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Wolfram Sang <wsa@kernel.org>