Revert "BUG/MAJOR: mux-quic: fix BUG_ON on empty STREAM emission"

Revert "BUG/MAJOR: mux-quic: fix BUG_ON on empty STREAM emission"

This reverts commit 98064537423fafe05b9ddd97e81cedec8b6b278d.

Above patch tried to fix a BUG_ON() occurence when MUX only emitted
empty STREAM frames via qcc_build_frms(). Return value of qcs_send() was
changed from the payload STREAM frame to the whole frame length.
However, this is invalid as this return value is used to ensure
connection flow-control is not exceeded on sending retry. This causes
occurence of BUG_ON() crash in qcc_io_send() as send-list is not
properly purged after QCS emission.

Reverts this incorrect fix. The original issue will be properly dealt in
the next commit.

This commit must be backported to 3.1 if reverted commit was already
applied on it.