mips: mt76x8: ddr_cal: Change types from u32 to int in dqs_find_min/max

This change is made to enable comparison of integer variables, which
might be negative in the next patch. No functional change is intended
in this patch.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Weijie Gao <weijie.gao@mediatek.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>