[MIPS] DECstation: Fix irq handling

[MIPS] DECstation: Fix irq handling

When I post a patch (commit f431baa55abf8adeed0c718b51deacbc151f58f1),
I just tried to not change behavior of existing codes, but it seems
dec/int-handler.S had been broken since its previous commit
937a801576f954bd030d7c4a5a94571710d87c0b.

The caller of plat_irq_dispatch do setup/restore TI_REGS($28), so
dec's plat_irq_dispatch should not do it, and there is no need to
adjust RA.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>