usb: bam: Use runtime PM APIs on BAM device for LPM

Simplify driver's handling of power management by
using its own runtimePM infrastructure instead of
invoking runtimePM APIs on parent USB controller
device.

Change-Id: Iafebc9b10d8327486b4899b520799e5e7b6629a7
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>