ixgbe(4): Fix VF build failure on i386 caused by r315333

SYSCTL_ADD_UQUAD on i386 expects a uint64_t pointer, and not an
unsigned long pointer. So change the data type of the original variable to
u64 (uint64_t) from unsigned long, since the statistics are intended to be
64 bits, anyway.

Reported by: Peter Jeremy (peter@rulingia.com)