mtd: cfi_cmdset_0002: Split do_write_oneword() to reduce function size

Reduce the size of do_write_oneword() by extracting a helper function
for the hardware access.

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