e1000e: helper functions for accessing EMI registers

The Extended Management Interface (EMI) registers are accessed by first
writing the EMI register offset to the EMI_ADDR regiter and then either
reading or writing the data to/from the EMI_DATA register.  Add helper
functions for performing these steps and convert existing EMI register
accesses accordingly.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>