Staging: comedi: addi_common.c: sparse cleanups

Now that we are properly marking the variable as __iomem, don't
cast it.

Also fix up some other sparse warnings.

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