mtd: rawnand: qcom: Add helper to check last code word

Add the qcom_nandc_is_last_cw() helper which checks if
the input cw index is the last one or not.

Signed-off-by: Md Sadre Alam <mdalam@codeaurora.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1614109141-7531-2-git-send-email-mdalam@codeaurora.org