net: stmmac: Fix error handling path in 'alloc_dma_tx_desc_resources()'

If the first 'kmalloc_array' within the loop fails, we should free what
as already been allocated, as done in all other error handling path.

Fixes: ce736788 ("net: stmmac: adding multiple buffers for TX")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>