dma: zynqmp: Minor code cleanup and fixes

- Added error check for platform_get_irq return value.
- Added condition for validating the bus-width permitted values.
- Initialize the dma attributes before reading them from the DT node
for avoiding the errors checks for each attribute.
- Framework expects bus-width in bytes. so, fixed it.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>