zlib/gunzip: Use WATCHDOG_RESET macro

As usually done in U-Boot, the watchdog_reset code is called via a
macro (WATCHDOG_RESET). In zlib.c this was done differently, by using
a function pointer which is initialized with WATCHDOG_RESET upon watchdog
usage or with NULL otherwise. This patch now uses the plain
WATCHDOG_RESET macros to call the function resulting in slightly smaller
U-Boot images and simpler code.

U-Boot code size reduction:

PowerPC board with watchdog support (lwmon5):
-> 80 bytes smaller image size

PowerPC board without watchdog support (sequoia):
-> 112 bytes smaller image size

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>