devices: console: simplify handle_input()
devices: console: simplify handle_input()

Instead of handling special conditions by setting a variable and
breaking from the loop into the error handling code, just return the
right error code directly.

BUG=None
TEST=serial console input works.

Change-Id: I02c9b8fd4bc64e4e0f16308d65cd05ace41c5756
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3565299
Reviewed-by: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm