post: fix I2C POST failure for devices in CONFIG_SYS_POST_I2C_IGNORES

Devices in CONFIG_SYS_POST_I2C_IGNORES list may be absent
and the rule is not to report I2C POST failure for devices
in this list. Currently this doesn't work since probing for
these devices isn't done and thus they are not marked as
successfully probed. Ignore optional devices when checking
for devices that didn't respond.

Signed-off-by: Anatolij Gustschin <agust@denx.de>