pc_sysfw: Plug memory leak on pc_fw_add_pflash_drv() error path

Harmless, because we the error inevitably leads to another, fatal one
in pc_system_flash_init(): PC system firmware (pflash) not available.
Fix it anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
