net: stmmac: rename GMAC_INT_DEFAULT_MASK for dwmac4

GMAC_INT_DEFAULT_MASK is written to the interrupt enable register.
In previous versions of the IP (e.g. dwmac1000), this register was
instead an interrupt mask register.
To improve clarity and reflect reality, rename GMAC_INT_DEFAULT_MASK
to GMAC_INT_DEFAULT_ENABLE.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>