watchdog: at91sam9_wdt: Fix WDT setup in at91_wdt_start()

This patch fixes the timer register setup in at91_wdt_start() to
correctly configure the register again. The input timeout value is
now in milli-seconds instead of seconds with the new watchdog API.
Make sure to take this into account and only use a max timeout
value of 16 seconds as appropriate for this SoC.

Also the check against a lower timeout value than 0 is removed. This
check makes no sense, as the timeout value is unsigned.

Signed-off-by: Stefan Roese <sr@denx.de>
Reported-by: Heiko Schocher <hs@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Tested on the taurus board:
Tested-by: Heiko Schocher <hs@denx.de>