rt2x00: convert rt2x00_rf_read return type

rt2x00: convert rt2x00_rf_read return type

This is a semi-automated conversion to change rt2x00_rf_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:\(\<rt2x00_rf_read\>(.*, .*\), &\(.*\));:\2 = \1);:' \
	drivers/net/wireless/ralink/rt2x00/rt*

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