prima: Fix un-initialized variable access

csrConstructRSNIe only declare rsn_ie buffer and calls
dot11fUnpackIeRSN using rsn_is as destination buffer.
This result use of var without initialization issue.

Fix is to initializes rsn_is buffer before using it.

Change-Id: I02959babcf4335e63623b45c899156af4e3bf53c
CRs-Fixed: 2303690