USB: g_file_storage: fix bug in DMA buffer handling

This patch (as963) fixes a recently-introduced bug.  The gadget
conversion removing DMA-mapped buffer allocation did not remove quite
enough code from the g_file_storage driver; DMA pointers were being
set to 0.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>