qcacld-3.0: Fix check_after_deref(REVERSE_INULL) issue

In __hdd_hard_start_xmit, Null-checking for skb under drop_pkt
suggests that it may be null, but it has already been dereferenced
on all paths.

Remove this Null-check for skb under drop_pkt as skb cannot be NULL.

Change-Id: I6c8e816652951a95cff26b26c48b8f39a096b095
CRs-Fixed: 2259256