linux/kernel.h: use 'short' to define USHRT_MAX, SHRT_MAX, SHRT_MIN

linux/kernel.h: use 'short' to define USHRT_MAX, SHRT_MAX, SHRT_MIN

The commit log of 44f564a4bf6a ("ipc: add definitions of USHORT_MAX and
others") did not explain why it used (s16) and (u16) instead of (short)
and (unsigned short).

Let's use (short) and (unsigned short), which is more sensible, and more
consistent with the other MAX/MIN defines.

As you see in include/uapi/asm-generic/int-ll64.h, s16/u16 are
typedef'ed as signed/unsigned short.  So, this commit does not have a
functional change.

Remove the unneeded parentheses around ~0U while we are here.

Link: http://lkml.kernel.org/r/1549156242-20806-1-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Zhang Yanmin <yanmin.zhang@intel.com>
Cc: Alex Elder <elder@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>