ARM: mvebu: Fix kernel hang in mvebu_soc_id_init() when of_iomap failed

When pci_base is accessed whereas it has not been properly mapped by
of_iomap() the kernel hang. The check of this pointer made an improper
use of IS_ERR() instead of comparing to NULL. This patch fix this
issue.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reported-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: stable@vger.kernel.org # v3.12+: af8d1c63: ARM: mvebu: Add support to get the ID and the revision of a SoC
Cc: stable@vger.kernel.org # v3.12+: 85e618a1: ARM: mvebu: Add quirk for i2c for the OpenBlocks AX3-4 board
Cc: stable@vger.kernel.org # v3.12+: 6cf70ae9: i2c: mv64xxx: Fix bus hang on A0 version of the Armada XP SoCs
Cc: stable@vger.kernel.org # v3.12+: f8b94beb: i2c: mv64xxx: Document the newly introduced Armada XP A0 compatible
Cc: stable@vger.kernel.org # v3.12+
Fixes: 930ab3d4 (i2c: mv64xxx: Add I2C Transaction Generator support)
Signed-off-by: Jason Cooper <jason@lakedaemon.net>