powerpc/interrupt: Use names in check_return_regs_valid()

trap->regs == 0x3000 is trap_is_scv()

trap 0x500 is INTERRUPT_EXTERNAL

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d48bf0184a1de185eb0ed3282247f8a294710674.1624632537.git.christophe.leroy@csgroup.eu