usb: gadget: printer: add req_match for printer function

Verify that a given usb_ctrlrequest is meant for printer function.
The following parts of the request are tested:

- bmRequestType:Data transfer direction
- bmRequestType:Type
- bmRequestType:Recipient
- bRequest
- wValue for bRequest 1 and 2
- wLength

Additionally, the request is considered meant for this function
iff the decoded interface number matches dev->interface.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>