regmap: cache: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

When all the registers are volatile(unlikely, but logically and mostly
will happen for some 'device' who has very few registers), then the
count will be euqal to 0, then kmalloc() will return ZERO_SIZE_PTR,
which equals to ((void *)16).

So this patch fix this with just doing the zero check before calling
kmalloc(). If the count == 0, so we can make sure that all the registers
are volatile, so no cache is need.

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