vme: tsi148: fix first DMA item mapping

This moves DMA mapping of the first list element to vme_list_add, the
same place where other elements mappings occur. This prevents extra
mapping or over-unmapping in the cases when vme_list_exec is called more
or less than one time respectively.

Also adds dma_mapping_error check.

Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Cc: Igor Alekseev <igor.alekseev@itep.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>