fm10k: Correct MTU for jumbo frames

Based on hardware testing, the host interface supports up to 15368 bytes
as the maximum frame size. To determine the correct MTU, we subtract 8
for the internal switch tag, 14 for the L2 header, and 4 for the
appended FCS header, resulting in 15342 bytes of payload for our maximum
MTU on jumbo frames.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>