USB: serial: remove zte_ev driver

The zte_ev driver is based on code (once) distributed by ZTE that still
appears to originally have been reverse-engineered and bolted onto the
generic driver.

A closer analysis of the zte_ev setup code reveals that it consists of
standard CDC requests (SET/GET_LINE_CODING and SET_CONTROL_LINE_STATE)
but unfortunately fails to get some of those right. In particular, as
reported by Lei Liu, it fails to lower DTR/RTS on close. It also appears
that the control requests lack the interface argument.

Since line control is already handled properly by the option driver, and
the SET/GET_LINE_CODING requests appears to be redundant (amounts to a
SET 9600 8N1) let's remove the redundant zte_ev driver.

Also move the remaining ZTE PIDs to the generic option modem driver.

Reported-by: Lei Liu <liu.lei78@zte.com.cn>
Signed-off-by: Johan Hovold <johan@kernel.org>