ath5k: clean up filter flags setting

The maze of if() statements in configure_filter is confusing.
Reorganizing it as a switch statement makes it more apparent what
is going on and reveals several suspicious settings.  This has no
functional changes, though it does remove some redundant flags
that are set earlier.

Also now that we can sleep, protect sc->filter_flags with the
sc lock.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>