ath5k: avoid and warn on potential infinite loop

If we are trying to interpolate a curve with slope == 0, the return
value will always be the y-coordinate.  In this code we are looping
until we reach a minimum y-coordinate on a line, which in the 0-slope
case can never happen, thus the loop never terminates.

The PCDAC steps come from the EEPROM and should never be equal, but
we should gracefully handle that case, so warn and bail out.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>