PM / OPP: fix condition for empty of_init_opp_table()

randconfig build reports the following error which is caused by
CONFIG_PM_OPP being unset.

  CC      arch/arm/mach-imx/mach-imx6q.o
arch/arm/mach-imx/mach-imx6q.c: In function ‘imx6q_opp_init’:
arch/arm/mach-imx/mach-imx6q.c:248:2: error: implicit declaration of function ‘of_init_opp_table’ [-Werror=implicit-function-declaration]

Fix the error by giving a more correct condition for empty
of_init_opp_table() implementation.

Reported-by: Rob Herring <robherring2@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>