cpu/sam0_common: ethernet: fix RX detection

[benpicco]

Contribution description

Detect RX of frame also when other bits of RSR are set.

Testing procedure

see #16298 (comment)

Issues/PRs references

fixes #16298

[dylad]

ACK

[benpicco]

Something is still very odd here:

2021-04-23 18:44:27,843 #  ping 2600:: -i 100000
2021-04-23 18:44:34,454 # 12 bytes from 2600::: icmp_seq=0 ttl=49 time=6605.967 ms
2021-04-23 18:46:15,315 # 12 bytes from 2600::: icmp_seq=1 ttl=49 time=7464.275 ms
2021-04-23 18:47:54,313 # 12 bytes from 2600::: icmp_seq=2 ttl=49 time=6459.737 ms
2021-04-23 18:47:54,314 # 
2021-04-23 18:47:54,316 # --- 2600:: PING statistics ---
2021-04-23 18:47:54,321 # 3 packets transmitted, 3 packets received, 0% packet loss
2021-04-23 18:47:54,326 # round-trip min/avg/max = 6459.737/6843.326/7464.275 ms

(this is on the border router)

On my Linux machine I get

% ping 2600:: -c 3
PING 2600::(2600::) 56 data bytes
64 bytes from 2600::: icmp_seq=1 ttl=49 time=130 ms
64 bytes from 2600::: icmp_seq=2 ttl=49 time=130 ms
64 bytes from 2600::: icmp_seq=3 ttl=49 time=129 ms

--- 2600:: ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 129.416/129.613/129.823/0.166 ms

[dylad]

@benpicco This might be another problem.
This fix is pretty simple and obvious so I think we should go with it first.
What do you think ?