i2400m/usb: wrap USB power saving in #ifdef CONFIG_PM

Current code was assuming PM was always enabled, which is not
correct. Code which accesses members in the struct usb_device that are
dependant on CONFIG_PM must be protected the same.

Reported by Randy Dunlap from a build error in the linux-next tree on
07/01/2009.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>