dwc_otg: fix potential sleep while atomic during urb enqueue

Fixes a regression introduced with eb1b482a. Kmalloc called from
dwc_otg_hcd_qtd_add / dwc_otg_hcd_qtd_create did not always have
the GPF_ATOMIC flag set. Force this flag when inside the larger
critical section.
Merge pull request #347 from P33M/rpi-3.6.y

dwc_otg: fix potential sleep while atomic during urb enqueue