xilinx_axidma: Do not set DMA .notify to NULL after notify

xilinx_axidma: Do not set DMA .notify to NULL after notify

If a stream notify function is not ready, it may re-populate the notify call-
back to indicate it should be re-polled later. This break in this usage, as
immediately following the notify() call, .notify is set to NULL. reverse the
ordering of the notify call and NULL assignment accordingly.

[PC: Reworked commit message]

Signed-off-by: Wendy Liang <[email protected]>
Signed-off-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Edgar E. Iglesias <[email protected]>