Writing beacon to device happen through multiple write command calls.
zd_usb_iowrite16v uses synchronous urb call and with multiple write
commands in row causes high CPU usage.

Make asynchronous zd_usb_iowrite16v_async available outside zd_usb.c
and use where possible.

This lower CPU usage from ~10% to ~2% on Intel Atom when running
AP-mode with 100 TU beacon interval.
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>