[WATCHDOG] gef_wdt.c: fsl_get_sys_freq() failure not noticed

fsl_get_sys_freq() may return -1 when 'soc' isn't found, but in
gef_wdt_probe() 'freq' is unsigned, so the test doesn't catch that.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>