smsc95xx: eliminate duplicate warnings on io failure

The register read/write functions already log a warning if
an access fails, so this patch removes the additional warnings
logged by callers that don't add any more information.

This patch makes the resulting driver smaller by not containing
as many warning strings.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>