staging: comedi: adv_pci1710: remove 'i8254_osc_base' from private data

This member of the private data is always set to I8254_OSC_BASE_10MHZ. Remove
it from the private data and just open code the value.

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>