mtd: nand: raw: atmel_nand: Use ATMEL_BASE_ECC directly

This is the only driver, and only one platform makes use of, setting
CONFIG_SYS_NAND_ECC_BASE. Reference ATMEL_BASE_ECC directly in this
case.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>