ARM: davinci: Use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
arch/arm/mach-davinci/devices-da8xx.c:255:8-14: WARNING: PTR_ERR_OR_ZERO can be used
arch/arm/mach-davinci/devices-da8xx.c:300:8-14: WARNING: PTR_ERR_OR_ZERO can be used
arch/arm/mach-davinci/dm646x.c:952:8-14: WARNING: PTR_ERR_OR_ZERO can be used
Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>