memory: brcmstb: dpfe: properly mask vendor error bits

We were printing the entire 32 bit register rather than just the lower
8 bits. Anything above bit 7 is reserved and may be any random value.

Fixes: 2f330caf ("memory: brcmstb: Add driver for DPFE")
Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>