post/board/lwmon5/gdc.c: Fix GCC 4.6 build warning

Fix:
gdc.c: In function 'gdc_test_reg_one':
gdc.c:66:6: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]

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