HID: i2c-hid: define i2c_hid_read_register() and use it

Handling simple read of device registers in __i2c_hid_command() makes it
too complicated and the need of special handling for the HID descriptor
register adds even more complexity. Instead, let's create simple
i2c_hid_read_register() helper on base of i2c_hid_xfer() and use 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>