qcom: ssr: Nullify subsys pointer in restart group if registration fails

When setting up subsys IRQs fails during the call to
subsys_register, null-ify the subsystem's pointer
in the restart group along with the other cleanup.
When/if the subsystem driver calls subsys_register
on the subsystem again, the subsystem's pointer will
be filled back.

Change-Id: I857cc7ccf16ce38c67bf9675ac16f8a5d4defcaa
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>