GridSeed docs are actually wrong, pll_BS is bit 31, and pll_BP is probably bit 30. If you look at the frequency tables in cgminer, you'll see that for frequencies >= 500 MHz, the highest bit is set.
Actually the gridseed version of cgminer sets bit30 low for 250/400/450mhz, and high for 500mhz and above just as they specified it in the docs.
Here's the frequency table from cpuminer (
https://github.com/gridseed/cpuminer/blob/master/gc3355.h#L51):
static const char *cmd_frequency[] = {
"55AAEF0005002001",
"55AAEF000500E001",
"55AAEF0005002002",
"55AAEF0005006082",
"55AAEF000500A082",
"55AAEF000500E082",
"55AAEF0005002083",
"55AAEF0005006083",
"55AAEF000500A083",
"55AAEF000500E083",
"55AAEF0005002084",
"55AAEF0005006084",
NULL
};
Note the 8 in the 2nd-last column, which corresponds to bit 31 (i.e. the high bit).