of: Minor simplification for the of_parse_phandles_with_args()

By using 'list++' in the beginning we can simplify the code a
little bit.

Suggested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>