net: stats: Allow TXTIME and statistics at the same time

Allow user to enable CONFIG_NET_PKT_TXTIME and
CONFIG_NET_PKT_TXTIME_STATS at the same time. This will increase
memory consumption but as the CONFIG_NET_PKT_TXTIME_STATS is
only meant to be enabled for debugging purposes this overhead
can be accepted.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>