net: stmmac: Make 'alloc_dma_[rt]x_desc_resources()' look even closer

'alloc_dma_[rt]x_desc_resources()' functions look very close.
Remove a useless initialization and use the same label name for error
handling path in order to get them even closer.

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>