orinoco_usb: implement fw download

This involves some refactorring of the common fw download code to
substitute ezusb versions of various functions.

Note that WPA-enabled firmwares (9.xx series) will not work fully with
orinoco_usb yet.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>