[TIPC]: Validate entire interface name when locating bearer to enable.

This fix prevents a bearer from being enabled using the wrong interface.
For example, specifying "eth:eth14" might enable "eth:eth1" by mistake.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Per Liden <per.liden@ericsson.com>