i2c: mux: reg: wrong condition checked for of_address_to_resource return value

of_address_to_resource return 0 on successful call but
devm_ioremap_resource is called only if it returns non-zero value

Signed-off-by: Lukasz Gemborowski <lukasz.gemborowski@nokia.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org