soc: ixp4xx/qmgr: fix invalid __iomem access

Sparse reports a compile time warning when dereferencing an
__iomem pointer:

drivers/soc/ixp4xx/ixp4xx-qmgr.c:149:37: warning: dereference of noderef expression
drivers/soc/ixp4xx/ixp4xx-qmgr.c:153:40: warning: dereference of noderef expression
drivers/soc/ixp4xx/ixp4xx-qmgr.c:154:40: warning: dereference of noderef expression
drivers/soc/ixp4xx/ixp4xx-qmgr.c:174:38: warning: dereference of noderef expression
drivers/soc/ixp4xx/ixp4xx-qmgr.c:174:44: warning: dereference of noderef expression

Use __raw_readl() here for consistency with the rest of the file.
This should really get converted to some proper accessor, as the
__raw functions are not meant to be used in drivers, but the driver
has used these since the start, so for the moment, let's only fix
the warning.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: d4c9e9fc ("IXP42x: Add QMgr support for IXP425 rev. A0 processors.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>