i2c: cbus-gpio: set atomic transfer callback

commit b12764695c3fcade145890b67f82f8b139174cc7 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>