mfd: omap-usb-tll: Fix channel count detection

Fix channel count detecion for REV2. Also, don't give up
if we don't recognize the IP Revision. We assume the default
number of channels (i.e. 3) for unrecognized IPs.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>