powerpc: cleanup hw_irq.h

SET_MSR_EE() is just use in this file and doesn't provide
any added value compared to mtmsr(). Drop it.

Add a wrtee() inline function to use wrtee/wrteei insn.

Replace #ifdefs by IS_ENABLED()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a28a20514d5f6df9629c1a117b667e48c4272736.1567068137.git.christophe.leroy@c-s.fr