[posix-netif] lower transmit log level for `OT_ERROR_DROP` (#7275)
[posix-netif] lower transmit log level for `OT_ERROR_DROP` (#7275)

This commit helps reduce posix-netif WARN logs when transmitting
messages to Thread.

There are certain ICMPv6 messages (e.g. MLDv2) that OpenThread will
filter when transmitting, so the error `OT_ERROR_DROP` is expected.

This commit also assumes that OpenThread knows what it's doing when it
choose to drop a message, thus using lower log level for
`OT_ERROR_DROP`.
srickardti/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,openthread/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread