mwifiex: simplify ad hoc join capability info

While preparing an ad-hoc start command, the capability info bitmap is
needlessly set from the command, and then the ESS bit cleared.

Change to set the bitmap directly without reference to the command.

Signed-off-by: James Cameron <quozl@laptop.org>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>