staging/wilc1000: move extern declarations to headers

'extern' declarations belong into a header file rather than
a .c file, to ensure that the definition matches the declaration.

This moves all declarations into a header file that seems
most appropriate for it.

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