config: correctly break string for prefix filter

config: correctly break string for prefix filter

This if statement does the following:

1. Copy the value of "config prefix_filter"
2. Break the string at "/"
3. Save the number after / as prefix length
4. Convert the address before / into in6_addr

However the extraneous "=" broke the second step and inet_pton
failed with addresses with mask. This commit removes the = thus
fixes the feature.

Signed-off-by: Tian Hao <haotia@gmail.com>