staging: comedi: ni_mio_common: make internal functions static

The functions ni_release_gpct_mite_channel() and ni_prime_channelgain_list()
are only referenced in this file. Make it static.

The function ni_release_gpct_mite_channel() is also only called
when PCIDMA is enabled. Move the #ifdef to quite a sparse warning
about the function not being used.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>