mtd: cfi_cmdset_0002: Split wait for write buffer completion sequence

Split wait for write completion from do_write_buffer() into separate
function.

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>
[vigneshr@ti.com: Reword commit message]
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>