Some write buffer functions are not used when FORCE_WORD_WRITE is set to 1.
So the compile warning messages are output if FORCE_WORD_WRITE is 1. To
resolve this disable the write buffer functions if FORCE_WORD_WRITE is 1.

Cc: Fabio Bettoni <fbettoni@gmail.com>
Co: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Tokunori Ikegami <ikegami.t@gmail.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>