drivers: staging: wilc1000: Add check for SPI availability

NULL pointer deference is observed in the wilc1000.ko module
with bus type SPI and when SPI is not ready.

Signed-off-by: Chandra S Gorentla <csgorentla@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>