libertas: remove priv->capability

This variable was once set to WLAN_CAPABILITY_SHORT_PREAMBLE and
there's no code that could change the variable to something else.
Therefore it seems this is not necessary :-)

Signed-off-by: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>