Revert "zlib: updated to v.1.2.3"

This reverts commit b201171f.

The commit caused problems for example when unpacking kernel images:

	   Uncompressing Kernel Image ... Error: inflate() returned -2
	   GUNZIP: uncompress, out-of-mem or overwrite error - must
	   RESET board to recover

Conflicts:

	include/u-boot/zlib.h
	lib_generic/zlib.c

Signed-off-by: Wolfgang Denk <wd@denx.de>