pwm: omap-dmtimer: Return -EPROBE_DEFER if no dmtimer platform data

If a pwm-omap-dmtimer is probed before the dmtimer it uses, the platform
data won't be set yet.

Fixes: ac30751d ("ARM: OMAP: pdata-quirks: Remove unused timer pdata")
Cc: <stable@vger.kernel.org> # 4.17+
Signed-off-by: David Rivshin <drivshin@allworx.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Ladislav Michl <ladis@linux-mips.org>
Tested-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>