USB: musb: Fix null pointer dereference issue

This patch fixes the following NULL pointer dereference issue.

Pointer 'request' returned from call to function 'next_request'
at line 748 may be NULL and may be dereferenced at line 792.

============
Code Snippet
============

748:  request = next_request(musb_ep);
785: if (dma && (csr & MUSB_RXCSR_DMAENAB)) {
	csr &= ~(MUSB_RXCSR_AUTOCLEAR
			| MUSB_RXCSR_DMAENAB
			| MUSB_RXCSR_DMAMODE);
	musb_writew(epio, MUSB_RXCSR,
		MUSB_RXCSR_P_WZC_BITS | csr);

792:	 request->actual += musb_ep->dma->actual_len;

Signed-off-by: Maulik Mankad <x0082077@ti.com>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>