eal/linux: fix return after alarm registration failure

eal/linux: fix return after alarm registration failure

When adding an alarm, if an error happen when registering
the common alarm callback, it is not considered as a major failure.
The alarm is then inserted in the list.
However it was returning an error code after inserting the alarm.

The error code is not set anymore to be consistent with the behaviour.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>