[MIPS] SMTC: Fix false trigger of debug code on single VPE.

Make smtc_setup_irq() update the list of interrupts which need to be
watched by the debug code itself.  Also there is no need to initialize the
IPI swint when running with a single VPE, so don't initialize it.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>