nvme-pci: clean up SMBSZ bit definitions

Define the bit positions instead of macros using the magic values,
and move the expanded helpers to calculate the size and size unit into
the implementation C file.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>