powerpc/powernv: Fix NVRAM sleep in invalid context when crashing

Similarly to opal_event_shutdown, opal_nvram_write can be called in
the crash path with irqs disabled. Special case the delay to avoid
sleeping in invalid context.

Fixes: 3b807033

 ("powerpc/powernv: Fix OPAL NVRAM driver OPAL_BUSY loops")
Cc: stable@vger.kernel.org # v3.2
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>