m68k: Implement ndelay() as an inline function to force type checking/casting

ndelay() is supposed to take an unsigned long, but if you define
ndelay() as a macro and the caller pass an unsigned long long instead
of an unsigned long, the unsigned long long to unsigned long cast is
not done and we end up with an "undefined reference to `__udivdi3'"
error at link time.

Fix that by making ndelay() an inline function and then defining dummy
ndelay() macro that redirects to the ndelay() function (it's how most
archs do to implement ndelay()).

Fixes: c8ee038b ("m68k: Implement ndelay() based on the existing udelay() logic")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
[geert: Remove comment now it is no longer a macro]
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>