spi: designware_spi: Use GENMASK

Replace numeric mask hexcodes with GENMASK macro
in designware_spi

Cc: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>