generic: b53: fix reverted error check in probe

b53_switch_detect returns value returned by b53_read8, which is 0 for
success. So fail (and return error) only if b53_switch_detect returned
something else than 0. This fixes supported and advertising being zeros
for MDIO access.

Cc: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 35534