watchdog: sp5100_tco: properly check for new register layouts

Commits 190aa430 (Add AMD Mullins platform support) and
cca118fa (Add AMD Carrizo platform support) enabled the
driver on a lot more devices, but the following commit missed
a single location in the code when checking if the SB800 register
offsets should be used. This leads to the wrong register being
written which in turn causes ACPI to go haywire.

Fix this by introducing a helper function to check for the new
register layout and use this consistently.

https://bugzilla.kernel.org/show_bug.cgi?id=114201
https://bugzilla.redhat.com/show_bug.cgi?id=1329910


Fixes: bdecfcdb (sp5100_tco: fix the device check for SB800
and later chipsets)
Cc: stable@vger.kernel.org (4.5+)
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>