qcacmn: Assert only when unmapped is true in rx_desc

The existing assertions for unmapped variable in
rx descriptor results in an undesired bug_on since
the unmapped value is 0 for active descriptors.

The fix is to modify this assertion so that bug_on will
be triggered only if the unmapped value is 1 in rx
desc.

Change-Id: I03b27e0e42ce3c42b9fa148da9258063b0209b08
CRs-Fixed: 2985248