libata: improve AC_ERR_DEV handling for ->post_internal_cmd

libata: improve AC_ERR_DEV handling for ->post_internal_cmd

->post_internal_cmd is simplified EH for internal commands.  Its
primary mission is to stop the controller such that no rogue memory
access or other activities occur after the internal command is
released.  It may provide error diagnostics by setting qc->err_mask
but this hasn't been a requirement.

To ignore SETXFER failure for CFA devices, libata needs to know
whether a command was failed by the device or for any other reason.
ie. internal command needs to get AC_ERR_DEV right.

This patch makes the following changes to AC_ERR_DEV handling and
->post_internal_cmd semantics to accomodate this need and simplify
callback implementation.

1. As long as the correct bits in the result TF registers are set,
   there is no need to set AC_ERR_DEV explicitly.  libata EH core
   takes care of that for both normal and internal commands.

2. The only requirement for ->post_internal_cmd() is to put the
   controller into quiescent state.  It needs not to set any err_mask.

3. ata_exec_internal_sg() performs minimal error analysis such that
   AC_ERR_DEV is automatically set as long as result_tf is filled
   correctly.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>