HID: i2c-hid: use "struct i2c_hid" as argument in most calls

The main object in the driver is struct i2c_hid so it makes more sense
to pass it around instead of passing i2c_client and then fetching
i2c_hid associated with it.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>