nvme: Use opcode directly in submit_io

nvme: Use opcode directly in submit_io

submit_io is an internal functions and all callers pass the correct
opcode in. Thus no need to reparse and select the right wrapper
function (nvme_read, nvme_write, ...). Instead just pass the opcode to
the generic nvme_io function.

Note, the compare was fixed in 9b6d3fca4289 ("submit_io(): fix "nvme
compare" to pass correct command") but missed the opportunity to
cleanup this part.

Signed-off-by: Tokunori Ikegami <ikegami.t@gmail.com>
[dwagner: reworded commit message]
Signed-off-by: Daniel Wagner <dwagner@suse.de>