qcacmn: Free buffer in ath_procfs_diag_read

A read buffer is allocated and used in
ath_procfs_diag_read and it is not freed
in one exit path which causes a memory leak

Free read buffer in all exit paths of
ath_procfs_diag_read

Change-Id: I1b2446a99f53c2f9130d395dd316eb4649eb24db
CRs-Fixed: 2352165