This is a preparatory for the serial-to-kfifo switch. kfifo understands
only scatter-gatter approach, so switch to that.

No functional change intended, it's just dmaengine_prep_slave_single()
inline expanded.
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240405060826.2521-11-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>