mhi: rmnet: remove debug message regarding invalid buff

The debug message does not accurately describe the reason
for rmnet to reschedule the request.

CRs-Fixed: 1016968
Change-Id: I49fa6a0dbdd9dec7a36f5705b9e8d27bc7352286
Signed-off-by: Tony Truong <truong@codeaurora.org>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>