net: ti: prueth_core: Fix hang when opening prueth ports on AM571 IDK

It is invalid to register same notifier block across multiple driver
instances and causes warn dump like [1]. It also causes board hang on
AM571 IDK when using ifconfig to bring up prueth interfaces.  Fix this
by registering separate notifier block per driver instance.

[1]:
[   27.441824] ------------[ cut here ]------------
[   27.489727] WARNING: CPU: 0 PID: 141 at kernel/notifier.c:26 notifier_chain_register+0x88/0xa0
[   27.638602] double register detected

While at that fix prueth_sw_unregister_notifiers() to use correct
unregister functions.

Fixes: f3142191

 ("net: ti: prueth_core: Add support for RSTP switch")
Reported-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>