mtd: spinand: winbond: Increase maximum frequency on an octal operation

The default number of dummy cycles is 16 in octal I/O mode (1S-8S-8S),
and with this default configuration the maximum frequency is higher than
what is being advertised. There are higher and lower frequency
possibilities, which involve making changes in the number of dummy
cycles through the VCR register. At this stage, let's just describe the
default configuration correctly. There should be no functional change.

Fixes: 1ac5ff2f ("mtd: spinand: winbond: Add octal support")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>