cam nvme: Add nvme_command_string

Replace nvme_cmd_string and nvme_opcode_string with a single function.
nvme_cmd_string was already using an sbuf around a caller-supplied
string, so use the same pattern for the entire command string.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D50651