bnxt: Do not compile on 32-bit platforms

The new bnxt_re driver doesn't compile on any of them (it uses writeq()
from the LinuxKPI, which isn't implemented there), and had already been
disconnected from the build on i386.

Reported by: Jenkins
Fixes: acd884dec99a ("RDMA/bnxt_re: Add bnxt_re RoCE driver")