Convert a long to rman_res_t, fixing a sign extension bug.

ahci.c had one signed long, which was passed into rman, rather than u_long.
After the switch of rman_res_t from size u_long to size uintmax_t, the sign
extension caused ranges to get messed up, and ahcich* to not attach.

There may be more signed longs used in this way, which will be fixed as they're
reported.

Reported by: pho