platform: auto: Initialize pwm as NULL

platform: auto: Initialize pwm as NULL

Initialize pwm variable to NULL and qualify it
before using PWM functions when pwm is not defined
in DT.

Bug 1926727

Change-Id: I2a7ab201645f4a4d3158bbfc281adea19ffd6cb5
Signed-off-by: Junghyun Kim <juskim@nvidia.com>
Reviewed-on: http://git-master/r/1485097
(cherry picked from commit b9f147d439b6f5b0bf28f5f4791ca0e6e862bdcd)
Reviewed-on: http://git-master/r/1485345
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Songhee Baek <sbaek@nvidia.com>
Reviewed-by: Gajanan Bhat <gbhat@nvidia.com>
Reviewed-by: Arun Shamanna Lakshmi <aruns@nvidia.com>