staging: comedi: mite.h: tidy up the mite channel register offsets

Convert the inline MITE_* functions, used to calculate the mite channel
register offsets, into simple macros.

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>