qcacmn: Add qdf_opmode_str()
It is often useful to print the human readable version of an operating
mode in a log message or some other user-facing string. Add a function
for mapping QDF_OPMODE to human readable strings, qdf_opmode_str(), in
QDF such that various consumers do not have to implement it themselves.
Change-Id: I8756fb2ea97b3b8c7af2633b07694ad1e74d6d3e
CRs-Fixed: 2331888