ddr: altera: Clean up rw_mgr_mem_calibrate_vfifo_center() part 3

Factor out common code from search_left_edge() and search_right_edge()
which checks whether searching for the window edge should stop. The
code is almost identical, so pull it into separate function and cater
for the minor differences.

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