parisc: Fix crash with nr_cpus=1 option

John David Anglin reported that giving "nr_cpus=1" on the command
line causes a crash, while "maxcpus=1" works.

Reported-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.18+