mlxsw: spectrum: Use only one function to create vFIDs

Simplify the code and use only one function for vFID creation /
destruction.

Unlike before, the function receives a FID index as its argument and not
a vFID index. Instead of passing 0, now one would need to pass 4K, which
is the first vFID.

This is the first step in creating a generic FID struct that will be
used for all three types of FIDs.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>