spi: atmel_spi: Use GENMASK

Replace numeric mask hexcodes with GENMASK macro
in atmel_spi

Cc: Bo Shen <voice.shen@atmel.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>
1 file changed