usb: serial: Add a check for current autosuspend-delay expiration

Upon autosuspend-delay expiration for the device, runtime pm thread
initiates run time suspend after checking device's power.last_busy
time. There is a possibility of race condition between this check
and device interface driver updating power.last_busy time which
results in interface suspend in the middle of ongoing transfer
on that interface.

CRs-Fixed: 381765
Change-Id: I1fc4f8d9d9e83ef3ef6c564ee76294a703230fad
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>