Revert "mwl8787: compile testmode.c only if NL80211_TESTMODE is defined."

This reverts commit 6820824786f479eae332c0364020a743c2c75fef.
mwl8787: conditionally compile testmode.o

Use config option to enable/disable it

Signed-off-by: Bob Copeland <me@bobcopeland.com>