msm: ipa: schedule nop only once

Make sure that sending a TX NOP packet is happening
only after sending a least one regular packet.

Change-Id: I2028249e3b6748d84d89dd0c48744a769eba2547
CRs-Fixed: 2173282
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>