staging: comedi: ni_mio_common: use 'reg' in ni_set_bitfield()

Use the passed 'reg' parameter to write to the STC registers instead
of duplicating the case symbol.

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>

diff --cc drivers/staging/comedi/drivers/ni_mio_common.c
Simple merge