staging: wlan-ng: Remove unused function hfa384x_docmd_async()

Remove unused function hfa384x_docmd_async.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Link: https://lore.kernel.org/r/20190712062807.9361-7-nishkadg.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>