Correct alloc_ and release_resource methods in thunder_pcie driver

• Avoid using BUS_ macros as bus_generic_ functions should be used instead.
• Fix mistaken device_t pointers in thunder_pcie_alloc_resource. Should use dev->parent method and allocate resource for child device

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5068