dpif-netdev: Refactor generic implementation

dpif-netdev: Refactor generic implementation

This commit refactors the generic implementation. The
goal of this refactor is to simplify the code to enable
"specialization" of the functions at compile time.

Given compile-time optimizations, the compiler is able
to unroll loops, and create optimized code sequences due
to compile time knowledge of loop-trip counts.

In order to enable these compiler optimizations, we must
refactor the code to pass the loop-trip counts to functions
as compile time constants.

This patch allows the number of miniflow-bits set per "unit"
in the miniflow to be passed around as a function argument.

Note that this patch does NOT yet take advantage of doing so,
this is only a refactor to enable it in the next patches.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Tested-by: Malvika Gupta <malvika.gupta@arm.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>