usb/core: consider link speed while looking at bMaxPower

The USB 2.0 specification says that bMaxPower is the maximum power
consumption expressed in 2 mA units and the USB 3.0 specification says
that it is expressed in 8 mA units.
This patch adds a helper function usb_get_max_power() which computes the
value based on config & usb_device's speed value. The the device descriptor
dump computes the value on its own.

Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>