igc: change default return of igc_read_phy_reg()

Static analysis reports this problem

igc_main.c:4944:20: warning: The left operand of '&'
  is a garbage value
    if (!(phy_data & SR_1000T_REMOTE_RX_STATUS) &&
          ~~~~~~~~ ^

phy_data is set by the call to igc_read_phy_reg() only if
there is a read_reg() op, else it is unset and a 0 is
returned.  Change the return to -EOPNOTSUPP.

Fixes: 208983f0 ("igc: Add watchdog")
Signed-off-by: Tom Rix <trix@redhat.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>