driver core: platform: Document return type of more functions

driver core: platform: Document return type of more functions

I can't always remember the return values of these functions, and so I
usually jump to the function to read the kernel-doc and see that it
doesn't tell me. Then I have to spend more time reading the code to jump
to the function that actually tells me the return values. Let's document
it here so that we don't all have to spend time digging through the code
to understand the return values.

Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200910060440.2302925-1-swboyd@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>