smsc75xx: add support for USB dynamic autosuspend

This patch adds support for USB dynamic autosuspend to the
smsc75xx driver.  This saves virtually no power in the USB
device but enables power savings in upstream hosts and
the host CPU.

Note currently Linux doesn't automatically enable this
functionality by default for devices so to test this:

 echo auto > /sys/bus/usb/devices/2-1.2/power/control

where 2-1.2 is the USB bus address of the LAN7500.

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