mtd: cqspi: Fix division by zero

Both dummy.nbytes and dummy.buswidth may be zero. By not checking
the later, it is possible to trigger division by zero and a crash.
This does happen with tiny SPI NOR framework in SPL. Fix this by
adding the check and returning zero dummy bytes in such a case.

Fixes: 38b0852b ("spi: cadence-qspi: Add support for octal DTR flashes")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Pratyush Yadav <p.yadav@ti.com>
[trini: Drop Pratyush's RB as his requested changes weren't made as
        Marek disagreed]