r8169: don't request firmware when there's no userspace.

The firmware is cached during the first successfull call to open() and
released once the network device is unregistered. The driver uses the
cached firmware between open() and unregister_netdev().

So far the firmware is optional : a failure to load the firmware does
not prevent open() to success. It is thus necessary to 1) unregister
all 816x / 810[23] devices and 2) force a driver probe to issue a new
firmware load.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Fixed-by: Ciprian Docan <docan@eden.rutgers.edu>
Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>