smsc75xx: expand check_ macros

These macros, while reducing the amount of code, hide flow control
and make the code more confusing to follow and review.  This patch
expands them.  It should have no functional effect on the driver.
Signed-off-by: NSteve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>