staging: comedi: acl7225b: cleanup acl7225b_di_insn_bits()

Use a local variable to hold the base register, 'reg', that is used
to read from the device.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>