ddr: altera: Clean up sdr_*_phase() part 4

ddr: altera: Clean up sdr_*_phase() part 4

Get rid of found_{begin,end} variables. Instead of breaking out
through all of the loops, just return when the begin/end of the
window is found and be done with it. Also clean up the trailing
conditional expression, which is now much easier.

Signed-off-by: Marek Vasut <marex@denx.de>