watchdog: bcm2835_wdt: Fix WDIOC_SETTIMEOUT handling

Users report about the unexpected behavior for setting timeouts above
15 sec on Raspberry Pi. According to watchdog-api.rst the ioctl
WDIOC_SETTIMEOUT shouldn't fail because of hardware limitations.
But looking at the code shows that max_timeout based on the
register value PM_WDOG_TIME_SET, which is the maximum.

Since 664a3923 ("watchdog: Introduce hardware maximum heartbeat
in watchdog core") the watchdog core is able to handle this problem.

This fix has been tested with watchdog-test from selftests.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217374
Fixes: 664a3923

 ("watchdog: Introduce hardware maximum heartbeat in watchdog core")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>