net: sfp: Unique GPIO interrupt names

Dynamically generate a unique GPIO interrupt name, based on the
device name and the GPIO name.  For example:

103:          0   sx1503q  12 Edge      sff2-los
104:          0   sx1503q  13 Edge      sff2-tx-fault

The sffX indicates the SFP the los and tx-fault are associated with.

v3:
- reverse Christmas tree new variable
- fix spaces vs tabs
v2:
- added net-next to PATCH part of subject line
- switched to devm_kasprintf()

Signed-off-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>