staging: comedi: ni_mio_common: use comedi_buf_read_samples()

Use comedi_buf_read_samples() to get the single samples from the async
buffer. The number of samples in the buffer was validated by the callers
so the error checking can be removed. In addition, the core will set the
COMEDI_CB_BLOCK event so it can also be removed.

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>