staging: comedi: dmm32at: remove forward declarations 2

Move the dmm32at_ns_to_timer() and dmm32at_setaitimer() functions
to remove the need for the remaining forward declarations.

Also, make dmm32at_setaitimer() static, it's only referenced in
this file.

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>