rtl8xxxu: Fix missing break in switch

Add missing break statement in order to prevent the code from falling
through to the default case.

Fixes: 26f1fad2 ("New driver: rtl8xxxu (mac80211)")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>