ARM: OMAP2+: Powerdomain: Fix unchecked dereference of arch_pwrdm

Commit 'cd8abed' "ARM: OMAP2+: Powerdomain: Remove the need to
always have a voltdm associated to a pwrdm" leads to the following
Smatch complaint:

arch/arm/mach-omap2/powerdomain.c:131 _pwrdm_register()
	 error: we previously assumed 'arch_pwrdm' could be null (see line 105)

So, fix the unchecked dereference of arch_pwrdm.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/powerdomain.c

@@ -128,6 +128,7 @@ skip_voltdm:
 	for (i = 0; i < pwrdm->banks; i++)
 		pwrdm->ret_mem_off_counter[i] = 0;
 
+	if (arch_pwrdm && arch_pwrdm->pwrdm_wait_transition)
 		arch_pwrdm->pwrdm_wait_transition(pwrdm);
 	pwrdm->state = pwrdm_read_pwrst(pwrdm);
 	pwrdm->state_counter[pwrdm->state] = 1;