e1000: force register write flushes to circumvent broken platforms

A certain AMD64 bridge (8132) has an option to turn on write combining
which breaks our adapter. To circumvent this we need to flush every write.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>