wl1251: halt the embedded CPU before loading firmware

After initial power-up, the embedded cpu is usually halted.  However,
if we down the interface and only do a soft reset before bringing
the interface back up, it will still be running and the firmware
loading code will bail out.  This change halts the CPU before loading
the firmware, enabling a second call to wl1251_boot() to succeed
without a hard reset.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>