sbsa_gwdt: Calculate timeout with 64-bit math

sbsa_gwdt: Calculate timeout with 64-bit math

Commit abd3ac7902fb ("watchdog: sbsa: Support architecture version 1")
introduced new timer math for watchdog revision 1 with the 48 bit offset
register.

The gwdt->clk and timeout are u32, but the argument being calculated is
u64. Without a cast, the compiler performs u32 operations, truncating
intermediate steps, resulting in incorrect values.

A watchdog revision 1 implementation with a gwdt->clk of 1GHz and a
timeout of 600s writes 3647256576 to the one shot watchdog instead of
300000000000, resulting in the watchdog firing in 3.6s instead of 600s.

Force u64 math by casting the first argument (gwdt->clk) as a u64. Make
the order of operations explicit with parenthesis.

Fixes: abd3ac7902fb ("watchdog: sbsa: Support architecture version 1")
Reported-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
Signed-off-by: Darren Hart <darren@os.amperecomputing.com>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: <stable@vger.kernel.org> # 5.14.x
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/7d1713c5ffab19b0f3de796d82df19e8b1f340de.1695286124.git.darren@os.amperecomputing.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>