staging: comedi: s626: remove I2CR and I2CW macros

The `I2CR` and `I2CW` macros expand to the I2C read and write addresses,
respectively.  They are only used in one place each and include the name
of a local variable `devpriv` in their expansion.

Get rid of the macros and expand them in place.

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