nvme: Tidy up transfer rate settings in XPT_GET_TRAN_SETTINGS.

• Replace a magic number with CTS_NVME_VALID_SPEC.

• Set the transport and protocol versions the same as for XPT_PATH_INQ.

Probably we shouldn't bother with setting the version in the 'spec'
member of ccb_trans_settings_nvme at all and use the transport
and/or protocol version field instead.

Reviewed by: chuck, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D40616