tpm_if.m: declare bus addresses as bus_size_t not bus_addr_t

Do like bus_space(9) does. This fixes the build on platforms where
bus_addr_t and bus_size_t are different (like i386 PAE).

Reviewed by: jhibbits
Fixes: c2e9c5bbf022 tpm: Refactor TIS and add a SPI attachment
Differential Revision: https://reviews.freebsd.org/D45287