watchdog: hpwdt (7/12): allow full range of timer values supported by hardware

The hpwdt timer is a 16 bit value with 128ms resolution.
Let applications use this entire range.

Signed-off-by: dann frazier <dannf@hp.com>
Acked-by: Thomas Mingarelli <Thomas.Mingarelli@hp.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>