parisc: Use num_present_cpus() in alternative patching code

When patching the kernel code some alternatives depend on SMP vs. !SMP.
Use the value of num_present_cpus() instead of num_online_cpus() to
decide, otherwise we may run into issues if and additional CPU is
enabled after having loaded a module while only one CPU was enabled.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # v6.1+