watchdog: it87_wdt: Keep WDTCTRL bit 3 unmodified for IT8784/IT8786

WDTCTRL bit 3 sets the mode choice for the clock input of IT8784/IT8786.
Some motherboards require this bit to be set to 1 (= PCICLK mode),
otherwise the watchdog functionality gets broken. The BIOS of those
motherboards sets WDTCTRL bit 3 already to 1.

Instead of setting all bits of WDTCTRL to 0 by writing 0x00 to it, keep
bit 3 of it unchanged for IT8784/IT8786 chips. In this way, bit 3 keeps
the status as set by the BIOS of the motherboard.

Watchdog tests have been successful with this patch with the following
systems:
  IT8784: Thomas-Krenn LES plus v2 (YANLING YL-KBRL2 V2)
  IT8786: Thomas-Krenn LES plus v3 (YANLING YL-CLU L2)
  IT8786: Thomas-Krenn LES network 6L v2 (YANLING YL-CLU6L)

Link: https://lore.kernel.org/all/140b264d-341f-465b-8715-dacfe84b3f71@roeck-us.net/

Signed-off-by: Werner Fischer <devlists@wefi.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20231213094525.11849-4-devlists@wefi.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>