libata: implement HORKAGE_1_5_GBPS and apply it to WD My Book

3Gbps is often much more prone to transmission failures.  It's usually
okay to let EH handle speed down after transmission failures but some
WD My Book drives completely shutdown after certain transmission
failures and after it only power cycling can revive them.  Combined
with the fact that external drives often end up with cable assembly
which is longer than usual and more likely to have intervening gender,
this makes these drives very likely to shutdown under certain
configurations virtually rendering them unusable.

This patch implements HOARKGE_1_5_GBPS and applies it to WD My Book
such that 1.5Gbps is forced once the device is identified.

Please take a look at the following bz for related reports.

  http://bugzilla.kernel.org/show_bug.cgi?id=9913

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>