staging/rdma/hfi1: Use device file minor to identify EPROM

When writing to the EPROM, the driver will always use the
"first" device.  This is incorrect for multiple cards.

Use the device file minor to determine the device to use.
Reject the generic device file.

Reviewed-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>