of_property_match_string returns an int; either an index from 0 or
greater if successful or negative on failure. Even it's very
unlikely that the DT CPU node contains multiple enable-methods
these checks should be fixed.

This patch was inspired by the work of Nick Desaulniers.

Link: https://lore.kernel.org/lkml/20230516-sunxi-v1-1-ac4b9651a8c1@google.com/T/
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://lore.kernel.org/r/20231228193903.9078-2-wahrenst@gmx.netReviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>