staging: comedi: adv_pci1710: change boardinfo 'fifo_half_size' to 'has_large_fifo'

The boards supported by this driver have a 4K or 1K FIFO that is used when reading
analog input samples when running an async command. The maximum number of samples
in the FIFO is half the FIFO size due to the 2 bytes/sample (12-bit resolution).

For aesthetics, change the 'fifo_half_size' member to a bit-field flag 'has_large_fifo'
and add a new member to the private data to hold the 'max_samples' available in the
FIFO. Refactor the board attach and pci1710_handle_fifo() accordingly.

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>