watchdog: orion: use 0 for unset heartbeat

If the heartbeat module param is not specified we would get an error
message

  watchdog: f1020300.watchdog: driver supplied timeout (4294967295) out of range
  watchdog: f1020300.watchdog: falling back to default timeout (171)

This is because we were initialising heartbeat to -1. By removing the
initialisation (thus letting the C run time initialise it to 0) we
silence the warning message and the default timeout is still used.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200313031312.1485-1-chris.packham@alliedtelesis.co.nz


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