ibmvnic: fix dma_mapping_error call

This patch fixes the dma_mapping_error call to use the correct dma_addr
which is inside the ibmvnic_vpd struct. Moreover, it fixes an uninitialized
warning regarding a local dma_addr variable which is not used anymore.

Fixes: 4e6759be

 ("ibmvnic: Feature implementation of VPD for the ibmvnic driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>