staging/wilc: fix Kconfig dependencies, second try

My first attempt to fix the Kconfig logic for wilc1000 was incomplete,
as it missed the case where SPI is built-in while SDIO is modular
and wilc1000 is configured as built-in in SPI mode (or vice versa),
which would still lead to a link failure.

This works around the problem by adding an intermediate Kconfig
symbol "WILC1000_DRIVER" that controls visibility of the SDIO and
SPI sub-drivers, so we can control the dependencies better.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9535ebc5 ("staging/wilc1000: fix Kconfig dependencies")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>