staging/wilc1000: split out bus specific modules

The SPI and SDIO specific code is now separate enough that
we just need to restructure the Makefile and Kconfig logic
a bit and export a couple of symbols from the common module
to have separate bus glue drivers.

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