igb: Fix duplicate include guard

The include guard "_E1000_HW_H_" is used by two separate header files in
two different drivers (e1000/e1000_hw.h and igb/e1000_hw.h). Using the
same include guard macro in more than one header file may cause
unexpected behavior from the compiler. Fix this by renaming the
duplicate guard in the igb driver.

Fixes: 9d5c8243 ("igb: PCI-Express 82575 Gigabit Ethernet driver")
Signed-off-by: Tom Seewald <tseewald@gmail.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>