Previous commit was data loss when writing to the DDR in burst smaller than c_DDR_BURST_LENGTH. Use wb_stb falling edge only on read cycles, use wb_we falling edge for write cycles.

git-svn-id: http://svn.ohwr.org/ddr3-sp6-core/trunk@85 739e5516-d4a2-47df-ba96-5610c1fa693f