spi: fix SPI_BIT_MASK so it always fits into 32-bits

On a 64-bit platform, ~0UL fills 64-bits, which causes SPI_BIT_MASK(32)
not to fit into 32 bits. This causes a warning when the result is assigned
to a 32-bit variable. Use ~0U instead to prevent this. This fixes:

drivers/spi/spi-gpio.c: In function 'spi_gpio_probe':
drivers/spi/spi-gpio.c:446:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>