r8169: rtl8169_set_speed_xmii cleanup.

Shorten chipset version test.

No functional change.

Careful readers will notice that the 'supports_gmii' flag is deduced
from the device PCI id. Though less specific than the chipset related
RTL_GIGA_MAC_VER_XY, it is good enough to detect a GMII deprieved 810x.
Some features push for a device specific configuration (improved jumbo
frame support for instance). 'supports_gmii' will follow this path
if / when the device PCI id test stops working.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>