openprom: fix warning

Fix following warnings:
openprom.c:510:2: warning: 'tmp' may be used uninitialized in this function [-Wmaybe-uninitialized]
openprom.c:503:3: warning: 'str' may be used uninitialized in this function [-Wmaybe-uninitialized]
openprom.c:459:8: warning: 'str' may be used uninitialized in this function [-Wmaybe-uninitialized]
openprom.c:422:7: warning: 'str' may be used uninitialized in this function [-Wmaybe-uninitialized]

Fixed by introducing PTR_ERR etc.
This simplified the code as a nice side effect.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>