spi: spi-qpic-snand: avoid memory corruption

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

The 'spi-qpic-nand' driver may cause memory corruption under some
circumstances. The first patch in the series changes the driver to
avoid that, whereas the second adds some sanity checks to the common
QPIC code in order to make detecting such errors easier in the future.