spi: spi-gpio: fix compilation warning on 64 bits systems

SPI_GPIO_NO_MOSI and SPI_GPIO_NO_MISO flags are type casted to unsigned
long, yet, they are to be stored in an unsigned int field in the
spi_gpio_platform_data structure.

This leads to the following warning during compilation on 64 bits systems:
warning: large integer implicitly truncated to unsigned type [-Woverflow]

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>