spi: sn-f-ospi: Fix division by zero

spi: sn-f-ospi: Fix division by zero

When there is no dummy cycle in the spi-nor commands, both dummy bus cycle
bytes and width are zero. Because of the cpu's warning when divided by
zero, the warning should be avoided. Return just zero to avoid such
calculations.

Fixes: 1b74dd64c861 ("spi: Add Socionext F_OSPI SPI flash controller driver")
Co-developed-by: Kohei Ito <ito.kohei@socionext.com>
Signed-off-by: Kohei Ito <ito.kohei@socionext.com>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://patch.msgid.link/20250206085747.3834148-1-hayashi.kunihiko@socionext.com
Signed-off-by: Mark Brown <broonie@kernel.org>