dma: xilinx: vdma: Do not disable interrupt in an intr handler

Interrupt was always disabled in interrupt handler, and
this caused some of halt issue, for example, when an error
interrupt was generated. So Change to enable interrupts
when a channel needs to be initialized, and disable interrupts
when a channel gets removed.

Signed-off-by: Hyun Kwon <hyunk@xilinx.com>