I noticed some weird behavior using DFU gadget on AT91SAM9260 which worked
1 of 10 times. The bug was due to the usb_gadget_poll() being called while
the previous call was still in progress : some USB calls may takes a lot of
time (ex : DFU erase-then-write), resulting in the poller to be triggered
again while still in the poll() function.

This patch adds a poll lock to the AT91 UDC driver.