mtd: rawnand: fsl_elbc: Fix detection when nand_scan_ident() has not selected ecc.mode

ecc.mode is set to 0 (aliased to NAND_ECC_NONE) either when function
nand_scan_ident() has not selected ecc.mode or when it selected it to none
ecc mode.

Distinguish between these two states by checking of node property
"nand-ecc-mode" which function nand_scan_ident() uses for filling ecc.mode.

This change fixes usage of none ecc mode if it is specified in DTS file.

Fixes: c9ea9019 ("mtd: rawnand: fsl_elbc: Use ECC configuration from device tree")
Signed-off-by: Pali Rohár <pali@kernel.org>