net sysfs: Print link speed as signed integer

Otherwise 4294967295 (MBit/s) (-1) will be printed when there is no link.
Documentation/ABI/testing/sysfs-class-net does not state if this shall be
signed or unsigned.
Also remove the now unused variable fmt_udec.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>