Ath5k: fix bintval setup

bintval is set to the initial value at .config_interface which is too
late, since it overwrites previously set value from .config. Move the
initialization to the .add_interface.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>