HID: hidraw: switch to using memdup_user

Instead of open-coding memory allocation and copying form user memory
sequence let's use memdup_user().

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