Fixed incomplete UBX CFG-MSG message

It is described in u-blox 6 Protocol Specification that CFG-MSG has length of payload = 8 and consists of:
U1 msgClass
U1 msgID
U1[6] rate for 6 targets

Currently, RTKLIB supports only 4 targets and generates messages such as this:
B5 62 06 01 06 00 02 10 00 00 00 00 1F D1

And even though u-blox receivers accept it, it is impossible to configure messages for two last targets, for example - SPI.

With this fix RTKLIB generates full message supporting all target interfaces:
b5 62 06 01 08 00 02 10 00 00 00 00 01 00 22 25