cpufreq: apple-soc: Fix an IS_ERR() vs NULL check

The of_iomap() function returns NULL if it fails.  It never returns
error pointers.  Fix the check accordingly.

Fixes: 6286bbb4 ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>