Fix build of ndis 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 use an
intermediate cast to uintptr_t to suppress it. In this case, the I/O
port range is effectively limited to the range of 0..65535.

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D15746