usb: misc: ks_bridge: Mark device busy during data xfer

EFS sync involves downloading file system image from external soc
and writing to local file system. Avoid unnecessary suspend/resume
in middle efs sync download by marking device busy whenever there is
data exchange.

Change-Id: Ib12d86188131420d2150b65876222457bf141262
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>