staging/wilc1000: unify device pointer

struct wilc has two pointers to store the device, one for sdio_func
and one for spi_device. By changing the pointer to a 'struct device',
we can simplify the logic and avoid a few #ifdefs.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>