mwifiex: Fix NL80211_TX_POWER_LIMITED

NL80211_TX_POWER_LIMITED was treated as NL80211_TX_POWER_AUTOMATIC,
which is the opposite of what should happen and can cause nasty
regulatory problems.

if/else converted to a switch without default to make gcc warn
on unhandled enum values.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>