generic: import patch lowering bitflip_threshold on SPI-NAND

generic: import patch lowering bitflip_threshold on SPI-NAND

Reporting an unclean read from SPI-NAND only when the maximum number
of correctable bitflip errors has been hit seems a bit late.
UBI LEB scrubbing, which depends on the lower MTD device reporting
correctable bitflips, then only kicks in when it's almost too late.

Set bitflip_threshold to 75% of the ECC strength, which is also the
default for raw NAND.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>