2) rework link state detection code & use it in POLLING mode
3) fix 2 bugs in link state detection code:
a) driver unable to detect link loss on bcm5721
b) on bcm570x chips (tested on bcm5700 bcm5701 bcm5702) driver fails
to detect link loss with probability 1/6 (solved in brgphy.c)
Devices working in TBI mode should not be affected by this change.
Approved by: glebius (mentor)
MFC after: 1 month