mtd: cqspi: Wait for transfer completion

Wait for the read/write transfer finish bit get actually cleared,
this does not happen immediately on at least SoCFPGA Gen5.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Pratyush Yadav <p.yadav@ti.com>