memory: brcmstb: dpfe: rename struct private_data

To avoid potential (future) conflicts with other data structures we
rename "struct private_data" to "struct brcmstb_dpfe_priv".

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>