Add support for all 4 possible channels to the ft2232_spi programmer

Add a check to validate the selected channel/interface, which not even
libftdi seems to do yet.
This patch changes default behavior: the new default channel/interface is A.
Also, this patch uses the word 'channel' in addition or in place of 'interface'
where possible without too much hassle because it is the term FTDI uses.

Corresponding to flashrom svn r1608.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
diff --git a/flashrom.8 b/flashrom.8
index 2519a78..ae6b398 100644
--- a/flashrom.8
+++ b/flashrom.8
@@ -555,7 +555,7 @@
 .SS
 .BR "ft2232_spi " programmer
 An optional parameter specifies the controller
-type and interface/port it should support. For that you have to use the
+type and channel/interface/port it should support. For that you have to use the
 .sp
 .B "  flashrom \-p ft2232_spi:type=model,port=interface"
 .sp
@@ -568,11 +568,11 @@
 and
 .B interface
 can be
-.BR A ", or " B .
+.BR A ", " B ", " C ", or " D .
 The default model is
 .B 4232H
 and the default interface is
-.BR B .
+.BR A .
 .sp
 If there is more than one ft2232_spi-compatible device connected, you can select which one should be used by
 specifying its serial number with the