USB: sdio: Fix a NULL pointer dereference

Fix a possible NULL pointer dereference of req in gsdio_write()
function.  Access req->actual only after checking req against
NULL.

CRs-Fixed: 492464
Change-Id: I82acb194b97d19f90a53ef4e5dccfdb5e16303ec
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>