i2400m_tx_skip_tail() needs to handle the special case of being called
when the tail room that is left over in the FIFO is zero.

This happens when a TX message header was opened at the very end of
the FIFO (without payloads). The i2400m_tx_close() code already marked
said TX message (header) to be skipped and this function should be
doing nothing.

It is called anyway because it is part of a common "corner case" path
handling which takes care of more cases than only this one.

The tail room computation was also improved to take care of the case
when tx_in is at the end of the buffer boundary; tail_room has to be
modded (%) to the buffer size. To do that in a single well-documented
place, __i2400m_tx_tail_room() is introduced and used.

Treat i2400m->tx_in == 0 as a corner case and handle it accordingly.

Found and diagnosed by Cindy H. Kao.
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>