Maximum number of mbuf frags is off-by-one for worst case scenario in mlx5en(4).

Inspecting the PRM no more than 0x3F data segments, DS, of size 16 bytes is
allowed.

Worst case scenario summary of DS usage:
Header is fixed: 2 DS
Maximum inlining: 98 => (98 - 2) / 16 = 6 DS
Remainder: 0x3F - 2 - 6 = 55 DS (mbuf frags)

Previously a value of 56 DS was used and this would work in the
normal case because not all inline data area was used up.

MFC after: 3 days
Approved by: re (marius)
Sponsored by: Mellanox Technologies