r8169: improve CPlusCmd handling

tp->cp_cmd is supposed to reflect the current value of the CplusCmd
register. Several (quite old) changes however directly change this
register w/o updating tp->cp_cmd. Also we have places in the code
reading this register where we could use the cached value.

In addition:
- Properly initialize tp->cmd with the register value.
- In rtl_hw_start_8169 remove one setting of PCIMulRW because it's
  set unconditionally anyway a few lines later.
- In rtl_hw_start_8168 properly mask out the INTT bits before
  setting INTT_1. So far we rely on both bits being zero.

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