watchdog: max6370: use __udelay() to avoid recursion

The udelay() function in lib/time.c contains a WATCHDOG_RESET()
call. So use __udelay() in max6370_wdt.c to prevent recursion.

Fixes: 0a095fc5 ("watchdog: Add MAX6370 watchdog timer driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>