watchdog: sp5100_tco: Write back the original value to reserved bits, instead of zero

In case of SP5100 or SB7x0 chipsets, the sp5100_tco module writes zero to
reserved bits. The module, however, shouldn't depend on specific default
value, and should perform a read-merge-write operation for the reserved
bits.

This patch makes the sp5100_tco module perform a read-merge-write operation
on all the chipset (sp5100, sb7x0, sb8x0 or later).

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43176


Signed-off-by: Takahisa Tanaka <mc74hc00@gmail.com>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: stable <stable@vger.kernel.org>