spi: dw: Actually mask interrupts

spi: dw: Actually mask interrupts

The designware spi driver unconditionally uses polling.

The comment to spi_hw_init() also states that the function should disable
interrupts.

According to the DesignWare DW_apb_ssi Databook, value 0xff in IMR enables
all interrupts. Since we want to mask all interrupts write 0x0 instead.

On the canaan k210 board, pressing the reset button twice to reset the
board will run u-boot. If u-boot boots Linux without having SPI interrupts
masked, Linux will hang as soon as interrupts are enabled, because of an
interrupt storm.

Properly masking the SPI interrupts in u-boot allows us to successfully
boot Linux, even after resetting the board.

Fixes: 5bef6fd79f94 ("spi: Add designware master SPI DM driver used on SoCFPGA")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
[Niklas: rewrite commit message]
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>