rge: Fix 32-bit powerpc build

Book-E powerpc has 64-bit bus_addr_t but only a 32-bit bus_size_t. Use
the right macros for maxsize and maxsegsize to fix the build.

Fixes: 4bf8ce037 ("if_rge: initial import of if_rge driver from OpenBSD.")
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D57794