staging: wilc1000: avoid use of 'g_spi' static variable

Instead of using static variable 'g_spi' move it as part of 'wilc'
struct. Also allocating the memory in the probe function and
free is taken care in wilc_netdev_cleanup().

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