eth: fbnic: fix `tx_dropped` counting

eth: fbnic: fix `tx_dropped` counting

Fix the tracking of rtnl_link_stats.tx_dropped. The counter
`tmi.drop.frames` is being double counted whereas, the counter
`tti.cm_drop.frames` is being skipped.

Fixes: f2957147ae7a ("eth: fbnic: add support for TTI HW stats")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250503020145.1868252-1-mohsin.bashr@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>