nvmet: looks at the passthrough controller when initializing CAP

For a passthru controller make cap initialization dependent on the cap of
the passthru controller, given that multiple Command Set support needs
to be supported by the underlying controller.  For that move the
initialization of CAP later so that it can use the fully initialized
nvmet_ctrl structure.

Fixes: ab5d0b38 (nvmet: add Command Set Identifier support)
Signed-off-by: Adam Manzanares <a.manzanares@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
[hch: refactored the code a bit to keep it more contained in passthru.c]
Signed-off-by: Christoph Hellwig <hch@lst.de>