msm: mdm: change where ssr_count is decremented

In fusion targets with more than one external modem, where
the modems are restarted together, ssr_count keeps track of
how many modems are still left to be completely restarted so
that any new SSR requests would be rejected until all modems
are restarted. Ssr_count should be decremented when the SSR
module sends the SUBSYS_AFTER_POWERUP notification instead of
during the powerup callback in order to be in sync with the
state of the SSR module.

Change-Id: I196f4937d929c54f475d251632f111404a2b3bf1
Signed-off-by: Joel King <joelking@codeaurora.org>
Signed-off-by: Sridhar Gujje <sgujje@codeaurora.org>