e1000e: convert head, tail and itr_register offsets to __iomem pointers

The Tx/Rx head and tail registers and itr_register are always at known
addresses based on the __iomem address at which the PCI region (from BAR 0)
is mapped and known offsets within the region for each of these registers.
Store and use the full address rather than just the region offset to reduce
unnecessary address calculations.  Also, change current u8 __iomem pointers
to void __iomem pointers.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>