usb: serial: Fix race between write after suspend and resume

wwan serial driver sets suspend flag when it goes to suspend.
Driver puts any write request to delayed queue during suspend
and submits the delayed write requests while resuming and clears
the suspend flag after that. This creates a race condition where
write request comes between queuing the delayed write request and
clearing the suspend flag.

CRs-Fixed: 360555
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Change-Id: Ib15b80bb7e289db66a6306b6147f058b5e78f3e5