cdc: Fix autoflush for FIFO < MPS

[tore-espressif]

Describe the PR
If CDC TX buffer size is configured to be less than bulk packet size, the autoflushing condition is never reached.

Changes:

1. TX buff is automatically flushed when TX FIFO is full

Additional context
Found out during espressif/esp-idf#9040

[tore-espressif]

This is in great extent reverting #547

[perigoso]

LGTM

[tore-espressif]

@PanRe @perigoso @hathach any update about this?

[hathach]

sorry for late response, I was too busy with other works. The reason for change request: although it has more code, the additional condition CFG_TUD_CDC_TX_BUFSIZE < BULK_PACKET_SIZE is constant expression and will help the compiler to optimize it out.

Therefore we don't have to run tu_fifo_full() when bufsize is larger than the packet size (and const expression also not executed at all in other case)

[hathach]

thank for the update