spi: spi-qpic-snand: enable 8 bits ECC strength

Merge series from Gabor Juhos <j4g8y7@gmail.com>:

This small patch set adds support for 8 bits ECC strength, which widens
the range of the usable SPI NAND chips with the driver.

The first one is a preparatory patch which adds some defines which
allows to avoid using magic values, and the second patch implements
the actual support.

The series should be integrated via the SPI tree, as that contains
prerequisite changes.