resource: Clean it up a bit

- Drop BUG_ON()s and do normal error handling instead, in
  find_next_iomem_res().

- Align function arguments on opening braces.

- Get rid of local var sibling_only in find_next_iomem_res().

- Shorten unnecessarily long first_level_children_only arg name.

Signed-off-by: Borislav Petkov <bp@suse.de>
CC: Andrew Morton <akpm@linux-foundation.org>
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