staging: comedi: addi_apci_3501: cleanup apci3501_reset()

Rename the CamelCase local variables.

Refactor the code a bit to remove the need for some of the local
variables.

Add a couple defines to the register map to help make the code
more concise.

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>