USB: u_bam: Remove runtimePM usage for BAM_DMUX

Driver has no implementation of suspend/resume for
BAM_DMUX transport. Hence, runtime_pm usage count
incremented on connect is decremented only on
disconnect. Since, runtime PM is not required for
BAM_DMUX, remove runtime PM APIs for the same.
Without this gadget's parent device may not be
SUSPENDED if gadget device's runtime state gets
changed to ACTIVE later which also marks gadget's
parent as ACTIVE due to active child.

Change-Id: I12b0b4a418a40ca7d0c70a99e4f2863e090789d7
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>