staging: comedi: ni_at_ao: introduce atao_select_reg_group()

To clarify the code a bit, introduce a helper function to set/clear
the GRP2WR bit in the CFG1 register. This bit selects which group of
registers are used for the registers that share the same I/O addresses.

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>