MINOR: quic: adjust quic_frame flag manipulation

MINOR: quic: adjust quic_frame flag manipulation

Replace a plain '=' operator by '|=' when setting quic_frame
QUIC_FL_TX_FRAME_LOST flag.

For the moment, this change has no impact as only two exclusive flags
are defined for quic_frame. On the edited code path we are certain that
QUIC_FL_TX_FRAME_ACKED is not set due to a previous if statement, so a
plain equal or a binary OR is strictly identical.

This change will be useful if new flags are defined for quic_frame in
the future. These new flags won't be resetted automatically thanks to
binary OR without explictly intended, which otherwise could easily lead
to new bugs.