bnx2x: add Big-Endian ethtool comment

There's a known issue that using `ethtool -e' flips the endianity of the
written data, i.e., using `ethtool -E' to dump eeprom image and than using
`ethtool -e' to re-write that same image will result in an image where
the data has the opposite endianity.
Sadly, this cannot be fixed as there are already various tools deployed
based on the endianity of the eeprom read/write.
Instead, a comment is added to the code to help explain why this is un-fixable.

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