HID: i2c-hid: no longer need raw access to HID descriptor structure

We can stop defining a union for HID descriptor data as we now only access
individual members of it by names and using proper types instead of
accessing by offset from the beginning of the data structure.

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>