staging: wilc1000: define undefined operation mode

This patch adds new define, IDLE_MODE to change comparison statement which
is wrong due to typecasting to null.

Signed-off-by: Johnny Kim <johnny.kim@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>