[LTC6952] Wrong datatype for VCO

[tfcollins]

linux/drivers/iio/frequency/ltc6952.c

Line 817 in f2ed46a

ret = of_property_read_u32(np, "adi,vco-frequency-hz",

VCO can reach 4.5 GHz therefore datatype should be a ULL. This just squeaks over the max value of a UL.