usb: dwc3: omap: remove unnecessary lock

the lock was only taken inside the hardirq
handler, which runs with IRQs disabled. There's
no chance of any race condition happening, even
on SMP machines. It's safe to remove that
spinlock.

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>