[NIU]: Cleanup PAGE_SIZE checks a bit

I get the following warning from a powerpc allyesconfig of current
mainline:

drivers/net/niu.c: In function 'niu_size_rbr':
drivers/net/niu.c:3113: warning: large integer implicitly truncated to unsigned type

PAGE_SIZE in this case is 64KB, so I don't quite get why gcc can't
tell that the line in question will never be reached.

I suggest the following instead, but I can unfortunately not do
anything but build test it.

Also, the driver does some other checks to make sure that PAGE_SIZE is
a power of two (BUILD_BUG_ON() in niu_init()), doesn't seem like that
could ever be untrue? Or are there really archs with non-power-of-two
PAGE_SIZE?

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: David S. Miller <davem@davemloft.net>