oxnas: fix qc_prep return in sata driver after kernel 4.14.200

oxnas: fix qc_prep return in sata driver after kernel 4.14.200

This fixes a regression after a kernel change in 4.14.200 [1] that
led to build failure on oxnas/ox820:

  drivers/ata/sata_oxnas.c:2238:13: error: initialization of
  'enum ata_completion_errors (*)(struct ata_queued_cmd *)'
  from incompatible pointer type
  'void (*)(struct ata_queued_cmd *)' [-Werror=incompatible-pointer-types]
    .qc_prep = sata_oxnas_qc_prep,
               ^~~~~~~~~~~~~~~~~~
  drivers/ata/sata_oxnas.c:2238:13: note:
  (near initialization for 'sata_oxnas_ops.qc_prep')

Our local driver is changed the same way as prototyped in the
kernel patch, i.e. return type is changed and AC_ERR_OK return
value is added.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=306a1c5b5683c1d37565e575386139a64bdbec6f

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit f6ca57e4f40528a8a0103c9f0e9647a2e11d10c3)