staging/mei: mei_wd_set_start_timeout should be static

mei_wd_set_start_timeout is only used from within wd.c
so remove its declaration from interface.h and mark
it static

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>