staging: pi433: use div_u64 for 64-bit division

I ran into this link error on an ARM OABI build:

drivers/staging/pi433/rf69.o: In function `rf69_set_frequency':
rf69.c:(.text+0xc9c): undefined reference to `__udivdi3'

No idea why I didn't see it with the default EABI configurations,
but the right solution here seems to be to use div_u64()
to get the external division implementation.

Fixes: 874bcba6 ("staging: pi433: New driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>