r8169: drop rtl_generic_op

Only two places are left where rtl_generic_op() is used, so we can
inline it and simplify the code a little.
This change also avoids the overhead of unlocking/locking in case
the respective operation isn't set.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>