HID: hiddev: reallocate hiddev's minor number

We need to store the minor number each drivers. In case of hidraw, the
minor number is stored stores in struct hidraw. But hiddev's minor is
located in struct hid_device.

The hid-core driver announces a kernel message which driver is loaded when
HID device connected, but hiddev's minor number is always zero. To proper
display hiddev's minor number, we need to store the minor number asked from
usb core and do some refactoring work (move from hiddev.c to hiddev.h) to
access hiddev in hid-core.

[jkosina@suse.cz: rebase on top of newer codebase]
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>