e1000e: do not modify tx_queue_len on link speed change

Previously the driver tweaked txqueuelen to avoid false Tx hang reports seen at half duplex.
This had the effect of overriding user set values on link change/reset. Testing shows that
adjusting only the timeout factor is sufficient to prevent Tx hang reports at half duplex.

This patch removes all instances of tx_queue_len in the driver.

Originally reported and patched by Franco Fichtner
CC: Franco Fichtner <franco@lastsummer.de>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>