Watchdog: fix clearing of the watchdog interrupt

The bits in BRIDGE_CAUSE are documented as RW0C - read, write 0 to
clear.  If we read the register, mask off the watchdog bit, and
write it back, we're actually clearing every interrupt which wasn't
pending at the time we read the register - and that is racy.

Fix this to only write ~WATCHDOG_BIT to the register, which means
we write as zero only the watchdog bit.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>