spi: spidev: Fix checkpatch issue

Fix the following checkpatch warnings.

  WARNING: Use #include <linux/uaccess.h> instead of <asm/uaccess.h>
  WARNING: braces {} are not necessary for any arm of this statement

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>