staging: fpgaboot: fix coccinelle warnings

drivers/staging/gs_fpgaboot/gs_fpgaboot.c:305:1-3: WARNING: PTR_RET can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: coccinelle/api/ptr_ret.cocci

CC: Insop Song <insop.song@gainspeed.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>