data-kernel: EMAC: Properly check desc availability in xmit

This change includes proper check for the no of descriptors
required for nr_frags in skb

Change-Id: If461b3830a2b0a21f12c60191186d42d22877101
CRs-Fixed: 2306538
Acked-by: Jagadeesh Babu Challagundla <jchallag@qti.qualcomm.com>
Signed-off-by: Sunil Paidimarri <hisunil@codeaurora.org>