firmware: Use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
drivers/firmware/efi/efi.c:610:8-14: WARNING: PTR_ERR_OR_ZERO can be used

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

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>