slim_ngd: Handle error flags before handling SENT flag

It's possible that error flags were set alongwith SENT flag for
TX transaction status. In such a scenario, error flag status should
be handled (and SENT flag should be cleared). Move error-flag
handling ahead of SENT flag handling to avoid interrupt being called
twice, and completion being set twice.

Change-Id: I13f28df0a0448790069bd103422dc1cd6c7ac408
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>