usb: gadget: printer: don't access file global pnp_string in function's code

In order to factor out a reusable f_printer, the function's code should
not use file global variables related to legacy printer gadget's
implementation.

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