HID: i2c-hid: explicitly code setting and sending reports

Instead of relying on __i2c_hid_command() that tries to handle all
commands and because of that is very complicated, let's define a
new dumb helper i2c_hid_xfer() that actually transfers (write and
read) data, and use it when sending and setting reports. By doing
that we can save on number of copy operations we have to execute,
and make logic of sending reports much clearer.

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>