firmware: fix capturing errors on fw_cache_init() on early init

register_pm_notifier() can technically fail, caputure this.
Note that register_syscore_ops() cannot fail given it just
adds an element to a linked list. This has been broken since
v3.7. Chances of this failing however are slim.

To improve code readability move the code folded under CONFIG_PM_SLEEP
into a helper.

Fixes: 07646d9c ("firmware loader: cache devices firmware during suspend/resume cycle")
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>