qcacld-3.0: Correct typo in csr_roam_joined_state_msg_processor

In csr_roam_joined_state_msg_processor, pRoamInfo->tx_stbc is
assigned twice because of a typo.
Assign pRoamInfo->rx_stbc as the value of
pUpperLayerAssocCnf->rx_stbc.

Change-Id: Ic90f6b486a50dcc3aca8cb7171a137a34319914d
CRs-Fixed: 2453956