wct4xxp: EC channel calculation in TONEDETECT assumes TE820.

Since r10290 "wct4xxp: Add support for TE820 and VPMOCT256." [1],
the TONEDETECT ioctl was not calculating the VPM channel correctly
on non TE820 cards. This fixes a regression first introduced in
2.6.0.

[1] http://svnview.digium.com/svn/dahdi?view=revision&revision=10290

Internal-Issue-ID: DAHLIN-302
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@10733 a0bf4364-ded3-4de4-8d8a-66a801d63aff
(cherry picked from commit abad4b447960bf3059d6a08485defa91ae05cf14)
wct4xxp: t4_serial_setup() was called more often than necessary.

The driver iterates through all the spans on a given device during assignment,
checking for unassigned spans, but it was erroneously testing the span on which
assigned was called.

This just removes some unexpected behavior and provides a slight performance
increase on load and does not impact the functionality of the driver as far as
I'm aware.

Reported-by: Doug Bailey <dbailey@digium.com>
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
(cherry picked from commit 9de213b1044c5e005d714b9cf00a367120d42392)