Fixed scan buffer increasing with WEXT

Fixed scan buffer increasing with WEXT

We can now handle up to 65535 byte result buffer which is the maximum
due to WEXT using 16-bit length field. Previously, this was limited to
32768 bytes in practice even through we tried with 65536 and 131072
buffers which we just truncated into 0 in the 16-bit variable.

This more or less doubles the number of BSSes we can received from scan
results.