sky2: jumbo frame regression fix

Remove unneeded check that caused problems with jumbo frame sizes.
The check was recently added and is wrong.
When using jumbo frames the sky2 driver does fragmentation, so
rx_data_size is less than mtu.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>