iConvert fxp(4) to new media and miibus KPI.

Functional changes:
o Move "Clear wakeup events" block down below if_attach(). This is

actually bugfix after initial conversion to projects/ifnet.

o In fxp_init() first set FXP_FLAG_RUNNING, then call mii_mediachg(),

otherwise fxp_miibus_statchg() would return at beginning.

o In fxp_miibus_statchg() don't return on !IFM_AVALID media, and

we won't call if_media_status().

Tested: Intel 82557 (phy DP83840, nsphy)
Tested: Intel 82558 (phy i82555, inphy)
Tested: Intel 82558B (phy i82555, inphy)
Tested: Intel 82559 (phy i82555, inphy)

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