AP_HAL_Linux: RCInput_SoloLink: better name to first field

Checking the time on the tcpdump capture, it matches the first fields
from the data:

$ tshark -n -c 4 -r  ~/tmp/solo/rc.pcap
    1   0.000000     10.1.1.1 → 10.1.1.10    UDP 68 5005 → 5005 Len=26
    2   0.019976     10.1.1.1 → 10.1.1.10    UDP 68 5005 → 5005 Len=26
    3   0.040046     10.1.1.1 → 10.1.1.10    UDP 68 5005 → 5005 Len=26
    4   0.059961     10.1.1.1 → 10.1.1.10    UDP 68 5005 → 5005 Len=26

From the previous commit (first 2 packets):

     5fa8 f441 3414 0500 73d7  dc05 dc05 dc05 db05 e803 e803 e803 f401
     73f6 f441 3414 0500 74d7  dc05 dc05 dc05 db05 e803 e803 e803 f401

0x0005143441f45fa8 - 0x0005143441f4f673 = 0x4E14 = 19988 (usec)

Which seems to approximately match for the other packets as well. We are
not using the field since we rather get the time when we receive it, but
at least use a better name.

libraries/AP_HAL_Linux/RCInput_SoloLink.h

@@ -39,10 +39,7 @@ private:
 
     union packet {
         struct PACKED {
-            /* changes at every packet */
-            uint32_t _unknown0;
-            /* apparently doesn't change: version? */
-            uint32_t _unknown1;
+            uint64_t timestamp_usec;
             le16_t seq;
             le16_t channel[8];
         };