USB: gadget: u_bam: Avoid extra context switch possibility in UL

U_BAM submits UL packets to BAM DMUX from WRITE complete notifier and
epout_completions by scheduling a workqueue. If RX flow control gets
kicked in, due to slow modem uplink, then USB RX requests are not
queued to hardware. And UL packets are then written to BAM DMUX from
work queue that is scheduled from WRITE COMPLETE notify of previous
request. BAM DMUX notify function is already called from workqueue.
This introduces additional context switches and any system level
work queue scheduling latencies may directly affect UL packets
submissions. This scenarios is more likely where modem uplink is
slow as compared to USB transfer rates which always is the case and
this change tries to remove any possibility of BAM_DMUX running out
of UL data (underruns) due to USB driver.

Change-Id: Ifbb50c72bb6a8bd99712d315c3f0e17ca7ea52dd
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>