em: remove M_HASHTYPE when RSS is not enabled

Summary:
Since "73fe85e486d2 tcp: store flowid info in syncache", inp_flowid can be set
if the incoming packet is not M_HASHTYPE_NONE. But this can introduce dummy
and duplicated flowid when a virtual interface set M_HASHTYPE_OPAQUE. This
change will let the upper layer know how to deal with software hash, with
benefits like inp_flowid can be set correctly and m_pkthdr.flowid can be set
correctly in output path.

This fix is similar to "20285cad7a55"

Reviewed by: kbowling
Differential Revision: https://reviews.freebsd.org/D55137