Since commit 97b2217ae5bc ("net/mlx4: advertise supported RSS hash
functions"), this PMD includes two similar-looking functions that convert
RSS hash fields between Verbs and DPDK formats.
This patch refactors them as a single two-way function and gets rid of
redundant helper macros.
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>