MIPS: DEC: Remove do_IRQ() call indirection

MIPS: DEC: Remove do_IRQ() call indirection

As from commit 8f99a1626535 ("MIPS: Tracing: Add IRQENTRY_EXIT section
for MIPS") do_IRQ() is not a macro anymore and can be invoked directly
from assembly code, as a tail call.  Remove the dec_irq_dispatch() stub
then and the indirection previously introduced with commit 187933f23679
("[MIPS] do_IRQ cleanup"), improving performance by reducing the number
of control flow changes and the overall instruction count, while fixing
a compiler's complaint about a missing prototype for said stub:

arch/mips/dec/setup.c:780:25: warning: no previous prototype for 'dec_irq_dispatch' [-Wmissing-prototypes]
  780 | asmlinkage unsigned int dec_irq_dispatch(unsigned int irq)
      |                         ^~~~~~~~~~~~~~~~

(which gets promoted to a compilation error with CONFIG_WERROR).

Fixes: 8f99a1626535 ("MIPS: Tracing: Add IRQENTRY_EXIT section for MIPS")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>