staging: wilc1000: fix sparse warning: symbol was not declared. Should it be static?

- Fixed a sparse warning:
  wilc_debugfs.c:32:10: warning: symbol 'WILC_DEBUG_LEVEL' was not declared. Should it be static?

Signed-off-by: Matias Mucciolo <mmucciolo@suteba.org.ar>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>