spi/atmel: Fix pointer to int conversion warnings on 64 bit builds

spi/atmel: Fix pointer to int conversion warnings on 64 bit builds

On 64 bit systems integers are generally still 32 bit but long values and
pointers are usually 64 bit. GCC warns when casting a 64 bit pointer into
a 32 bit integer so cast to a long instead in order to avoid warnings.

Signed-off-by: Mark Brown <broonie@linaro.org>