res_fax: allow 2400 transmission rate according to v.27ter standard

A previous set of patches (see: ASTERISK-22790 & ASTERISK-23231) made it so
a v.27 modem was not allowed to have a minimum transmission rate of 2400 bits
per second. This reverts all or some of those patches since according to the
v.27ter standard a rate of 2400 bits per second is also supported.

One of the original patches also added 9600 bits per second support for v.27.
This patch also removes that since v.27ter only supports 2400/4800 bits per
second.

Also, since Asterisk specifically supports v.27ter the enum was renamed to
better reflect this.

ASTERISK-24955 #close
Reported by: Matt Jordan

Change-Id: I4b9dfb6bf7eff08463ab47ee1a74224f27cae733