staging: comedi: s626: local functions should not be exposed globally

The 'ResetADC' function is prototyped as a static but is missing
the static tag in it's definition. The function is only referenced
in this file so add the static tag to the definition.

This quiets a sparse warning about:

warning: symbol 'ResetADC' was not declared. Should it be static?

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