leds: pwm: Fix for deferred probe in DT booted mode

We need to make sure that the error code from devm_of_pwm_get() is the one
the module returns in case of failure.
Restructure the code to make this possible for DT booted case.
With this patch the driver can ask for deferred probing when the board is
booted with DT.
Fixes for example omap4-sdp board's keyboard backlight led.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>