MIPS: Make virt_addr_valid() return bool

virt_addr_valid() really returns a boolean value, but currently uses an
integer to represent it. Switch to the bool type to make it clearer that
we really are returning a true or false value.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: linux-mips@vger.kernel.org