usb: dwc2: gadget: sparse warning of context imbalance

sparse was giving the following warning:
        warning: context imbalance in 's3c_hsotg_ep_enable'
	                - different lock contexts for basic block

we were returning ENOMEM while still holding the spinlock.
The sparse warning was fixed by releasing the spinlock before return.

Cc: <stable@vger.kernel.org> # v3.17
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>