FROMLIST: pstore-ram: fix NULL reference when used with pdata

When using platform-data (not DT), we get an OOPS, because drvdata is
only initialized after we try to use it.

This addresses my comments made on the upstream submission here:

https://patchwork.kernel.org/patch/7980651/



Fixes boot on Chrome OS systems, including the Pixel 2.

Change-Id: I97360edf2ce61c83dc543cb6c169f3287e2dae4b
Fixes: b1d1b718 ("FROMLIST: pstore-ram: add Device Tree bindings")
Signed-off-by: Brian Norris <briannorris@google.com>