powerpc: Fix potential compile error irqs_disabled_flags

irqs_disabled_flags is #defined in linux/irqflags.h when
CONFIG_TRACE_IRQFLAGS_SUPPORT is enabled.  64 and 32 bit always have
CONFIG_TRACE_IRQFLAGS_SUPPORT enabled so just remove
irqs_disabled_flags.

This fixes the case when someone needs to include both linux/irqflags.h
and asm/hw_irq.h.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>