dmaengine: jz4780: Fix error handling/signedness issues

There are a some signedness bugs such as testing for < 0 on unsigned
return values. Additionally there are some cases where functions which
should return NULL on error actually return a PTR_ERR value which can
result in oopses on error. Fix these issues.

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>