stmmac: ipq806x: Return error values instead of pointers

Typically we return error pointers when we want to use those
pointers in the non-error case, but this function is just
returning error pointers or NULL for success. Change the style to
plain int to follow normal kernel coding styles.

Cc: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>