intel: handle unused assignments

Remove variables that were storing a return value from a register
read or other read, where the return value wasn't used. Those
conversions to remove the lvalue of the assignment should be safe
because the readl memory mapped reads are marked volatile and
should not be optimized out without an lvalue (I suspect a very
long time ago this wasn't guaranteed as it is today).

These changes are part of a separate patch to make it easier to review.

Warnings Fixed:
.../intel/e100.c:2596:9: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]
.../intel/ixgb/ixgb_hw.c:101:6: warning: variable ‘icr_reg’ set but not used [-Wunused-but-set-variable]
.../intel/ixgb/ixgb_hw.c:277:6: warning: variable ‘ctrl_reg’ set but not used [-Wunused-but-set-variable]
.../intel/ixgb/ixgb_hw.c:952:15: warning: variable ‘temp_reg’ set but not used [-Wunused-but-set-variable]
.../intel/ixgb/ixgb_hw.c:1164:7: warning: variable ‘mdio_reg’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:132:6: warning: variable ‘ret_val’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:380:6: warning: variable ‘icr’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:2378:6: warning: variable ‘signal’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:2374:6: warning: variable ‘ctrl’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:2373:6: warning: variable ‘rxcw’ set but not used [-Wunused-but-set-variable]
.../intel/e1000/e1000_hw.c:4678:15: warning: variable ‘temp’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>