msm: ipa: do not send zero length packet to IPA

Fail the operation of ipa_tx_dp() in case skb size is zero.
This is done as IPA will not generate a status packet for zero
length packet and to avoid a possible memory leak.

Change-Id: I8abab2e32e5937a86802d2d5c73c4e7243775ea5
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>