staging: wilc1000: use struct to pack join parameters for FW

Refactor code to use struct to construct the join parameters. Avoid use
of extra buffer before sending to FW instead directly pass the struct
pointer.

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