net: ipa: remove endpoint delay mode feature

A "delay mode" feature was put in place to work around a problem
that was observed during development of the upstream IPA driver.  It
used TX endpoint "delay mode" in order to prevent transmitting
packets toward the modem before it was ready.

A race condition that would explain the problem has long since been
fixed, and we have concluded that the "delay mode" feature is no
longer required.  So get rid of it.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>