Do not leak a ioremap()'d cookie around, unmaping it in case of errors

Fixes: cef4bafc ("soc: brcmstb: add SoC driver to brcmstb")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>