libertas: configurable beacon interval

Requires firmware version 5.110.19.p0 or newer, available here:
http://dev.laptop.org/pub/firmware/libertas/



Signed-off-by: Ashish Shukla <ashishs@marvell.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>