[IA64] Fix processor_get_freq

The core cpufreq code doesn't appear to understand returning -EAGAIN
for the get() function of the cpufreq_driver.  If PAL_GET_PSTATE returns
-1, such as when running on Xen, scaling_cur_freq is happy to return
4294967285 kHz (ie. (unsigned)-11).  The other drivers appear to return
0 for a failure, and doing so gives me the max frequency from
scaling_cur_frequency and "<unknown>" from cpuinfo_cur_frequency.  I
believe that's the desired behavior.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>