libertas: if_spi, driver for libertas GSPI devices

Add initial support for libertas devices using a GSPI interface.  This has
been tested with the 8686.

GSPI is intended to be used on embedded systems. Board-specific parameters are
required (see libertas_spi.h).

Thanks to everyone who took a look at the earlier versions of the patch.

Signed-off-by: Colin McCabe <colin@cozybit.com>
Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>