net: dhcp6: Fix OPT_BOOTFILE_PARAM parsing

RFC 5970 states that OPT_BOOTFILE_PARAM (option 60) can be
multiple parameters that start with a 16-bit length field followed
by the parameter. For example:
[ param-len 1 (16-bits) ] [ parameter 1 (variable length) ]

This fix ensure we're considering "param-len 1" in the parsing.

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>