mtd: nand: Fix a docs build warning

Commit 0b4773fd (mtd: nand: Drop unused cached programming support)
removed the "cached" parameter from nand_write_page(), but did not update
the kerneldoc comments, creating this docs build warning:

  ./drivers/mtd/nand/nand_base.c:2751: warning: Excess function parameter 'cached' description in 'nand_write_page'

Remove the offending line so we can have a little peace and quiet.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>