watchdog: sp5100: Fix definition of EFCH_PM_DECODEEN3

EFCH_PM_DECODEEN3 is supposed to access DECODEEN register bits 24..31,
in other words the register at byte offset 3.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Fixes: 887d2ec5 ("watchdog: sp5100_tco: Add support for recent FCH versions")
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Link: https://lore.kernel.org/r/20200910163109.235136-1-linux@roeck-us.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>