fm10k: Modify tunnel length header check when offloading

The FM10000 host interface can only support up to 184 bytes when
performing tunnel offloads. Because of this, a check was added to
prevent the driver from attempting to feed a header to the hardware too
big for it to parse. Make this check a little more robust by calculating
the inner L4 header length based on whether it is TCP or UDP.

Cc: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>