dmaengine: dw-edma: Fix readq_ch() return value truncation

Previously, readq_ch() did a 64-bit readq(), but truncated the result by
storing it in the u32 "value".  Change "value" to u64 to avoid the
truncation.

Note: the method is currently unused, so the bug hasn't caused any problem
so far.

Fixes: 04e0a39f ("dmaengine: dw-edma: Add writeq() and readq() for 64 bits architectures")
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>