spi: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

Since we cannot make sure the 'n' will always be none zero here, and
then if either equal to zero, the kzalloc() will return ZERO_SIZE_PTR,
which equals to ((void *)16).

So this patch fix this with just doing the zero check before calling
kzalloc().

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>