pstore: Properly assign mem_type property

If mem-type is specified in the device tree
it would end up overriding the record_size
field instead of populating mem_type.

As record_size is currently parsed after the
improper assignment with default size 0 it
continued to work as expected regardless of the
value found in the device tree.

Simply changing the target field of the struct
is enough to get mem-type working as expected.

Fixes: 9d843e8f

 ("pstore: Add mem_type property DT parsing support")
Cc: stable@vger.kernel.org
Signed-off-by: Luca Stefani <luca@osomprivacy.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221222131049.286288-1-luca@osomprivacy.com