dmaengine: pxa: fix double counting of the hw descriptors

In the Linux kernel, the following vulnerability has been resolved:

dmaengine: pxa: fix double counting of the hw descriptors

pxad_alloc_desc() was converted from

kzalloc(struct_size(sw_desc, hw_desc, nb_hw_desc), GFP_NOWAIT)

to kzalloc_flex(), which sets the __counted_by() counter sw_desc->nb_desc
itself - but only where the compiler has __builtin_counted_by_ref(), so
from gcc 15.1 or clang 22.1 on. The loop below it still increments
nb_desc, which makes it come out doubled there and correct elsewhere.

nb_desc is what pxad_free_desc() iterates over and what
set_updater_desc() indexes from, so set it explicitly and drop the
increment. The error path has to lower it to the number of descriptors
allocated so far, otherwise pxad_free_desc() would free entries that were
never allocated.