Staging: comedi: addi_amcc_s5933.h: sparse cleanup

Mark a variable static that does not need to be global.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>