Fix build of bxe with base gcc on i386

Casting from rman_res_t to a pointer results in "cast to pointer from
integer of different size" warnings with base gcc on i386, so print
these without casting. The kva field of struct bxe_bar is of type
vm_offset_t, which can be 32 or 64 bit, so cast it to uintmax_t before
printing.

Reviewed by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D15733