HID: i2c-hid: rework i2c_hid_get_report() to use i2c_hid_xfer()

Explicitly prepare command for i2c_hid_get_report() which makes the logic
clearer and allows us to get rid of __i2c_hid_command() and related command
definitions.

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>