staging: comedi: ni_mio_common: fix M Series ni_ai_insn_read() data mask

For NI M Series cards, the Comedi `insn_read` handler for the AI
subdevice is broken due to ANDing the value read from the AI FIFO data
register with an incorrect mask.  The incorrect mask clears all but the
most significant bit of the sample data.  It should preserve all the
sample data bits.  Correct it.

Fixes: 817144ae ("staging: comedi: ni_mio_common: remove unnecessary use of 'board->adbits'")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>