iwlwifi: pcie: make sure prph_info is set when treating wakeup IRQ

In some rare cases when the HW is in a bad state, we may get this
interrupt when prph_info is not set yet.  Then we will try to
dereference it to check the sleep_notif element, which will cause an
oops.

Fix that by ignoring the interrupt if prph_info is not set yet.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211219132536.0537aa562313.I183bb336345b9b3da196ba9e596a6f189fbcbd09@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>