bus: ixp4xx: return on error in ixp4xx_exp_probe()

This code was intended to return an error code if regmap_read() fails
but the return statement was missing.

Fixes: 1c953bda ("bus: ixp4xx: Add a driver for IXP4xx expansion bus")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210807230016.3607666-1-linus.walleij@linaro.org

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>