rt2x00: convert rt2x00usb_register_read return type

This is a semi-automated conversion to change rt2x00usb_register_read
to return the register contents instead of passing them by value,
resulting in much better object code. The majority of the patch
was done using:

sed -i 's:\(\<rt2x00usb_register_read\>(.*, .*\), &\(.*\));:\2 = \1);:' \
    -i 's:\(\<rt2500usb_register_read\>(.*, .*\), &\(.*\));:\2 = \1);:' \
    -i 's:\(\<rt2500usb_register_read_lock\>(.*, .*\), &\(.*\));:\2 = \1);:' \
	 drivers/net/wireless/ralink/rt2x00/*

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>