The message printing in nvs.c is mixed with pr_*() and
printk(), but with no prefix and also no pr_fmt() defined.

Introduce pr_fmt() and use pr_*() macros to replace printk(),
to generate a unified format string for prefix.
Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>