staging: comedi: ni_mio_common: protect register write overflow

staging: comedi: ni_mio_common: protect register write overflow

[ Upstream commit 1cbca5852d6c16e85a21487a15d211195aacd4a1 ]

Fixes two problems introduced as early as
commit 03aef4b6dc12  ("Staging: comedi: add ni_mio_common code"):
(1) Ensures that the last four bits of NISTC_RTSI_TRIGB_OUT_REG register is
    not unduly overwritten on e-series devices.  On e-series devices, the
    first three of the last four bits are reserved.  The last bit defines
    the output selection of the RGOUT0 pin, otherwise known as
    RTSI_Sub_Selection.  For m-series devices, these last four bits are
    indeed used as the output selection of the RTSI7 pin (and the
    RTSI_Sub_Selection bit for the RGOUT0 pin is moved to the
    RTSI_Trig_Direction register.
(2) Allows all 4 RTSI_BRD lines to be treated as valid sources for RTSI
    lines.

This patch also cleans up the ni_get_rtsi_routing command for readability.

Fixes: 03aef4b6dc12  ("Staging: comedi: add ni_mio_common code")
Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>