Cast everything to something longer than 32 bits so the sample mask
doesn't get truncated to 32 bits.

Without this, 3x3 NICs transmitting at an MCS rate whose rix (rate
index) in the rate table is > 31 end up returning errors, as the
sample rate code doesn't think the rate is set in the rate table.

Tested:

• AR9380, STA, speaking 3x3 to an AP