staging: comedi: c6xdigio: introduce c6xdigio_get_encoder_bits()

The 24-bit encoder value is read using 3-bits in the status register. The
data register is banged between each read of the status register to advance
the bits.

Introduce a helper function to handle this and remove the union encvaluetype
and struct encbitsbyte.

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>