In Striding RQ, each WQE serves multiple packets
(hence called Multi-Packet WQE, MPWQE).
The size of a MPWQE is constant (currently 256KB).
Upon a ringparam set operation, we calculate the number of
MPWQEs per RQ. For this, first it is needed to determine the
number of packets that can reside within a single MPWQE.
In this patch we use the actual MTU size instead of ETH_DATA_LEN
for this calculation.
This implies that a change in MTU might require a change
in Striding RQ ring size.
In addition, this obsoletes some WQEs-to-packets translation
functions and helps delete ~60 LOC.