bnx2x: Fix linearization for encapsulated packets

Due to FW constraints, driver must make sure that transmitted SKBs will
not be too fragmented, or in the case that they are - that each 'window'
of fragments passed to the FW would contain at least an mss worth of data.

For encapsultaed packets the calculation is wrong, since it ignores the
inner headers in the calculation of the headers' length.
This could lead to a FW assertion in case of a too-fragmented encapsulated
packet.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>