mac80211: remove redundant initialization to pointer 'hdr'

The pointer hrd is being initialized with a value that is never read
and re-assigned a little later, hence the initialization is redundant
and can be removed.

Cleans up clang warning:
net/mac80211/tx.c:1924:24: warning: Value stored to 'hdr' during its
initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>