sky2: receive checksum refactoring

Break the largish case for handling receive checksum into a separate
function, and if there is a problem use dev_XXX routines to
show which hardware is the problem.

Turn one corner case into a BUG().  This only happens if the driver
is expecting one behavior but the chip does the old behavior;
only ever saw this when bringing up a new chip type and driver
was buggy.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>