sfc: Fix memory BAR release call on error path

Match pci_request_region() with pci_release_region(), not
release_mem_region().

From: Steve Hodgson <shodgson@solarflare.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>