net: stmmac: Check return value of alloc_dma_desc_resources()

alloc_dma_desc_resources() returns an error value and the next line
actually checks for it, so assign the return value properly.

Found by the coverity scanner.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>