dma: imx-dma: fix signedness bug

mxdmac->channel was unsigned, so check (imxdmac->channel < 0) for
failed imx_dma_request_by_prio() made no sence.  Explicitly check
signed values.
Also, fix uninitialzed use of ret.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>