msm: ipa3: Change to avoid the assert channel not allocated scenario

When the modem channel not running if try to flow control the channel
it will return not valid channel. Adding change to avoid the assert
in this error condition.

Change-Id: Ia9c32fbc11814e5fef3b7fbcc165439b5d03b2fa
Signed-off-by: Ashok Vuyyuru <avuyyuru@codeaurora.org>