dmaengine: qcom-adm: fix wrong calling convention for prep_slave_sg

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

dmaengine: qcom-adm: fix wrong calling convention for prep_slave_sg

The calling convention for pre_slave_sg is to return NULL on error and
provide an error log to the system. Qcom-adm instead provide error
pointer when an error occur. This indirectly cause kernel panic for
example for the nandc driver that checks only if the pointer returned by
device_prep_slave_sg is not NULL. Returning an error pointer makes nandc
think the device_prep_slave_sg function correctly completed and makes
the kernel panics later in the code.

While nandc is the one that makes the kernel crash, it was pointed out
that the real problem is qcom-adm not following calling convention for
that function.

To fix this, drop returning error pointer and return NULL with an error
log.