libsas: Don't BUG when connecting two expanders via wide port

When a device is connected to an expander, the discovery process goes through
sas_ex_discover_dev to figure out what's attached to the phy.  If it is the
case that the phy being discovered happens to be the second phy of a wide link
to an expander, that discover_dev function will incorrectly call
sas_ex_discover_expander, which creates another sas_port and tries to attach the
other sas_phys to the new port, thus triggering a BUG.  The correct thing to do is
to check the other ex_phys of the expander to see if there's a sas_port for this
sas_phy, and attach the sas_phy to the existing sas_port.

This is easily triggered if one enables the phys of a wide port between
expanders one by one.

This second version of the patch fixes a small regression in the case where
all the phys show up at once and we accidentally try to attach to a port
that hasn't been created yet.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>