hw/dmax/xlnx_dpdma: fix handling of address_extension descriptor fields

hw/dmax/xlnx_dpdma: fix handling of address_extension descriptor fields

The DMA descriptor structures for this device have
a set of "address extension" fields which extend the 32
bit source addresses with an extra 16 bits to give a
48 bit address:
 https://docs.amd.com/r/en-US/ug1085-zynq-ultrascale-trm/ADDR_EXT-Field

However, we misimplemented this address extension in several ways:
 * we only extracted 12 bits of the extension fields, not 16
 * we didn't shift the extension field up far enough
 * we accidentally did the shift as 32-bit arithmetic, which
   meant that we would have an overflow instead of setting
   bits [47:32] of the resulting 64-bit address

Add a type cast and use extract64() instead of extract32()
to avoid integer overflow on addition. Fix bit fields
extraction according to documentation.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Cc: qemu-stable@nongnu.org
Fixes: d3c6369a96 ("introduce xlnx-dpdma")
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
Message-id: 20240428181131.23801-1-adiupina@astralinux.ru
[PMM: adjusted commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>