nvmem: jz4780-efuse: fix build warnings on ARCH=x86_64 or riscv

kbuild-robot did find a type error in the min(a, b)
function used by this driver if built for x86_64 or riscv.

Althought it is very unlikely that this driver is built
for those platforms it could be used as a template
for something else and therefore should be correct.

The problem is that we implicitly cast a size_t to
unsigned int inside the implementation of the min() function.

Since size_t may differ on different compilers and
plaforms there may be warnings or not.

So let's use only size_t variables on all platforms.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: srinivas.kandagatla@linaro.org
Cc: prasannatsmkumar@gmail.com
Cc: malat@debian.org
Cc: paul@crapouillou.net
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200310132257.23358-15-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>