usbnet: smsc75xx: don't recover device if suspend fails in system sleep

If suspend callback fails in system sleep context, usb core will
ignore the failure and let system sleep go ahead further, so
this patch doesn't recover device under this situation.

Also add comments on this case.

Acked-by: David S. Miller <davem@davemloft.net>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>