dm: core: Don't inline dev_read...() calls with of-platdata

At present if these calls are used with of-platdata, a confusing error is
produced, referring to a function not actually called by the code causing
the problem.

Fix this by not inlining, so that the error mentions the dev_read_...()
function and it is more obvious what is going on.

Signed-off-by: Simon Glass <sjg@chromium.org>