staging: comedi: s526: refactor gpct registers

The S526 board has 4 general purpose counter/timers. The current
defines used for the registers of these devices do not provide much
clarity in what they are.

Refactor the code to add some clarity.

Replace the current register defines with macros that return the
correct offset based on the counter 'channel'. Introduce a some helper
functions to handle reading/writing the LSB/MSB registers so that
they are always accessed in the correct oreder.

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