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>

drivers/staging/pi433/rf69.c

@@ -238,7 +238,7 @@ int rf69_set_frequency(struct spi_device *spi, u32 frequency)
 	do_div(f_step, 524288); //  524288 = 2^19
 
 	// check input value
-	f_max = f_step * 8388608 / factor;
+	f_max = div_u64(f_step * 8388608, factor);
 	if (frequency > f_max)
 	{
 		dev_dbg(&spi->dev, "setFrequency: illegal input param");