memory: brcmstb: dpfe: optimize generic_show()

We can pass a (struct priv_data *) to generic_show() rather than a
(struct device *). For two of the three callers of this function, the
change means one less call to dev_get_drvdata(). For the third caller,
it makes no difference.

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