soc: brcmstb: Fix error path for unsupported CPUs

In case setup_hifcpubiuctrl_regs() returns an error, because of e.g:
an unsupported CPU type, just catch that error and return instead of
blindly continuing with the initialization. This fixes a NULL pointer
de-reference with the code continuing without having a proper array of
registers to use.

Fixes: 22f7a911 ("soc: brcmstb: Correct CPU_CREDIT_REG offset for Brahma-B53 CPUs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>