watchdog: imx2_wdt: avoid to ping before resume back

Since watchdog_ping_work is not freezable so that it maybe scheduled before
imx2_wdt_resume where watchdog clock enabled, hence, kernel will hang in
imx2_wdt_ping without clock, and then watchdog reset happen. Add clk_is_on
to prevent the above case by ignoring ping until watchdog driver resume
back indeed.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1621012875-22667-1-git-send-email-yibin.gong@nxp.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>