powerpc/mpic: Check if cpu_possible() in mpic_physmask()

In mpic_physmask() we loop over all CPUs up to 32, then get the hard
SMP processor id of that CPU.

Currently that's possibly walking off the end of the paca array, but
in a future patch we will change the paca array to be an array of
pointers, and in that case we will get a NULL for missing CPUs and
oops. eg:

  Unable to handle kernel paging request for data at address 0x88888888888888b8
  Faulting instruction address: 0xc00000000004e380
  Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  NIP .mpic_set_affinity+0x60/0x1a0
  LR  .irq_do_set_affinity+0x48/0x100

Fix it by checking the CPU is possible, this also fixes the code if
there are gaps in the CPU numbering which probably never happens on
mpic systems but who knows.

Debugged-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>