mips: mt76x8: ddr_cal: Rename dqs_test_valid() to dqs_test_error()

This function returns "-1" (true) upon error. So the function name does
not match its implementation which is confusing. This patch renames the
function to dqs_test_error() which makes the code easier to read.

Also change the return type to bool and return "true" or "false".

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>