mac80211: make debugfs mcs set entry reflect 16 bits

The MCS set is 16 bits so when debugging ensure the full 16 bits
are represented. Current reading would make you think its only
8 bits.

Cc: johannes@sipsolutions.net
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>