staging: gs_fpgaboot: fix a compiler warning with make W=2

The below warning is fixed:

drivers/staging/gs_fpgaboot/gs_fpgaboot.c: In function ‘gs_load_image’:
drivers/staging/gs_fpgaboot/gs_fpgaboot.c:196:58: warning: declaration of ‘file’ shadows a global declaration [-Wshadow]
drivers/staging/gs_fpgaboot/gs_fpgaboot.c:45:14: warning: shadowed declaration is here [-Wshadow]

by renaming file function argument of gs_load_image with fw_file.

Signed-off-by: Devendra Naga <devendranaga4@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>