mtd: spinand: macronix: Fix ECC Status Read

According to datasheet, the upper four bits are reserved or used for
reflecting the ECC status of the accumulated pages. The error bits
number for the worst segment of the current page is encoded on lower
four bits. Fix it by masking the upper bits.

This same issue has been already fixed in the linux kernel by:
"mtd: spinand: macronix: Fix ECC Status Read"
(sha1: f4cb4d7b46f6409382fd981eec9556e1f3c1dc5d)

Apply the same fix in the U-Boot driver.

Signed-off-by: Haolin Li <li.haolin@qq.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>