m68k: define __kernel_size_t as unsinged int again

Commit ddc94378d changed the definition of __kernel_size_t
from unsigned int to unsigned long.

It is true that it fixed warnings on some crosstools
but it increased warnings on the others.

The problem is that we cannot see consistency in terms of
the typedef of __kernel_size_t on M68K architecture.

However, I'd like to suggest to have __kernel_size_t to be
unsigned int again.

Rationale:

[1] Linux Kernel defines __kernel_size_t on M68K as unsigned int.
    Let's stick to the Linux's way.

[2] We want to build boards with popular pre-built toolchains,
    not the one locally-built by indivisuals.
    I think m68-linux-gcc which can be downloaded from www.kernel.org
    is the candidate for our _recommended_ toolchains.

With this patch, all the m68k boards can be built without any warnings.

Give it a try with the following crosstools:

https://www.kernel.org/pub/tools/crosstool/files/bin/x86_64/4.6.3/
x86_64-gcc-4.6.3-nolibc_m68k-linux.tar.xz

or

https://www.kernel.org/pub/tools/crosstool/files/bin/x86_64/4.9.0/
x86_64-gcc-4.9.0-nolibc_m68k-linux.tar.xz

(The latter is newer.)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jason Jin <Jason.jin@freescale.com>