NTB: correct ntb_peer_spad_addr and ntb_peer_spad_read comment typos

The comment for ntb_peer_spad_addr and ntb_peer_spad_read
incorrectly referred to peer doorbell register and local
scratchpad register.

Signed-off-by: Wesley Sheng <wesley.sheng@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>