powerpc/5200: Build fix for mpc52xx watchdog timer code

mpc52xx_gpt_wdt_setup is defined as 0, which causes the following build
failure with gcc 4.5, since it's built with -Werror.

arch/powerpc/platforms/52xx/mpc52xx_gpt.c:761:3: error: statement with no effect

Changing it to a static inline fixes the problem.

Reported-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>