rt2x00: convert rt2x00_desc_read return type

This is a semi-automated conversion to change rt2x00_desc_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:\(\<rt2x00_desc_read\>(.*, .*\), &\(.*\));:\2 = \1);:' \
    -i 's:\(\<_rt2x00_desc_read\>(.*, .*\), &\(.*\));:\2 = \1);:' \
drivers/net/wireless/ralink/rt2x00/rt*

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