usb: gadget: gr_udc: improve platform_device variable name

Rename struct platform_device pointers from ofdev to pdev for clarity,
while at that, also use platform_set/get_drvdata() helpers.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>