usb: gadget: udc: renesas_usb3: lock for PN_ registers access

This controller disallows to change the PIPE until reading/writing
a packet finishes. However. the previous code is not enough to hold
the lock in some functions. So, this patch fixes it.

Fixes: 746bfe63 ("usb: gadget: renesas_usb3: add support for Renesas USB3.0 peripheral controller")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>