soc: qcom: qmi_interface: Do not print error for 0 pkt

Add a check for the zero size flow control packet when handling data
from the socket. This packet is expected and should not be printed as
an error.

Change-Id: I74a9edf752cb5f186ef133ae9d8b12cc93ee1b18
Signed-off-by: Chris Lew <clew@codeaurora.org>