Staging: bcm: Fix a coding style error reported by checkpatch.pl

Tool checkpatch.pl reported the following error: extern struct class *bcm_class;"
declaration not allowed in .c files. Therefore, I move this declaration into
the header "headers.h" file.

Signed-off-by: Kevin McKinney <klmckinney1@gmail.com>
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>