staging: comedi: ni_mio_common: tidy up ni_gpct_write_register()

The 'bits' value passed to this function is properly set by the callers
so the BUG_ON() can never occur when writing to the NITIO_G[01]_INT_ENA
and NITIO_G01_RESET registers. Remove the BUG_ON() checks.

For aesthetic, also remove the static const variables. They don't add
any additional clarity.

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>