axivdma: xilinx: Remove irq_dispose_mapping() as it is not required

This patch removes the call irq_dispose_mapping() as in the code
we are not mapping the interrupt using create_mapping().  This
patch fixes the kernel crash when unloading this driver module.

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>