PCI: endpoint: pci-epf-vntb: Fix sparse build warning for epf_db

Use epf_db[i] dereference instead of readl() because epf_db is
in memory allocated by dma_alloc_coherent(), not I/O.

Remove useless/duplicated readl() in the process.

Link: https://lore.kernel.org/r/20221102141014.1025893-7-Frank.Li@nxp.com


Signed-off-by: Frank Li <frank.li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>