soc: bcm: brcmstb: Don't leak device tree node reference

of_find_node_by_path() acquires a reference to the node returned by it
and that reference needs to be dropped by its caller. soc_is_brcmstb()
doesn't do that, so fix it.

[treding: slightly rewrite to avoid inline comparison]

Fixes: d52fad26 ("soc: add stubs for brcmstb SoC's")
Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>