mtd: nand: omap_gpmc: fix error handling

"err" was an unsigned variable, causing negative error codes to turn
into positive values, which are interpreted as an amount of succesfully
corrected bitflips (and thus not an error).

In particular, this resulted in that if the elm reports uncorrectable
errors (-EBADMSG), the MTD layer (and UBI) falsely succeeded.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Reviewed-by: Stefan Roese <sr@denx.de>