staging: comedi: c6xdigio: tidy up c6xdigio_pwm_write()

The 10-bit pwm value (2-498) is written by banging 2-bits in the data register.

Refactor this function to remove the need for the union pcmcmdtype and struct
pwmbitstype.

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>