staging: wilc1000: remove use of 'terminated_handle' static variable

Remove use of 'terminated_handle' variable and set the 'hif_drv' to
NULL once it's free.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>