revert removal of __ETHTOOL_LINK_MODE_LAST macro

Since Linux 5.0 the __ETHTOOL_LINK_MODE_LAST macro is no longer available.
commit e728fdf06289 ("net: phy: improve definition of __ETHTOOL_LINK_MODE_MASK_NBITS")
replaced it with __ETHTOOL_LINK_MODE_MASK_NBITS.

In v0.3.6.17.2 the vendor driver replaced __ETHTOOL_LINK_MODE_LAST+1
with 0xFFFFFFFF. This reverts that, because we want to use the proper
solution that upstream drivers use, namely
__ETHTOOL_LINK_MODE_MASK_NBITS.