mwifiex: fix missing break on IEEE80211_STYPE_ACTION case

The IEEE80211_STYPE_ACTION case is missing a break in the switch
statement, causing it to fall through to the default case that
reports a debug message about an unknown frame subtype. Fix this
by adding in the missing break statement.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>