xyz-modem: Fix timeout loop waiting with WATCHDOG

Commit 2c77c0d6524eb ("xyz-modem: Change getc timeout loop waiting")
fixes the loop delay when using a hw watchdog, assuming that watchdog
kicking is taken care of by getc(). But the xyzmodem driver tries to
do a getc only after confirming that a character is available like below:
	while (!tstc()) {
		till timeout;
	}
	if (tstc())
		*c = getc();

and getc() does a watchdog reset only if it fails to see a character.
In this case, getc() always sees a character and never does a
watchdog reset. So to make sure that watchdog doesn't get reset
while loading the file, do a watchdog reset just before starting the
image loading.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>