virtio: console: simplify process_transmit_queue
virtio: console: simplify process_transmit_queue

The error path reproduces the code outside of the match arm, so it can
be removed.

BUG=b:228912920
TEST=cargo build

Change-Id: Ibd485a52441be03501041bc4136bab9293c7066b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3600165
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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