ramips: don't clobber vlans with remapped vid on mt7530/762x switches

Avoid overwriting vlan entries with remapped vid in later iterations of
the vlan enumeration loop of mt7530_apply_config().

Fix the problem by refactoring the code to first reset the entire table,
then reprogram only vlans with members to prevent overwriting configured
vlans with unconfigured ones.

Fixes FS#1147, FS#1341

Signed-off-by: Jo-Philipp Wich <jo@mein.io>