USB: u_bam: Reserve skb headroom only if needed

Check if skb headroom is not available, then
reserve headroom.

Incase this is not done, it leads to skb_panic
because skb head would become lesser than skb
data.

CRs-Fixed: 644290
Change-Id: I9754abd01833d264ad0a73152c8a49be19364246
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>