net: davinci_emac: fix NULL check after pointer dereference

NULL check is made after the pointer dereference. This patch
fixes this issue.

Signed-off-by: Vishwas Srivastava <vishu.kernel@gmail.com>
CC: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>