stmmac: fix a typo in the macro used to mask the mmc irq

This patch fixes the name of the macro used to mask the
mmc interrupt: erroneously it was used: MMC_DEFAUL_MASK.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>