e1000e: make lost bits explicit

For more than 15 years this code has passed in a request for a page and
masked off that page when read/writing. This code has been here forever,
but FIELD_PREP finds the bug when converted to use it. Change the code
to do exactly the same thing but allow the conversion to FIELD_PREP in a
later patch. To make it clear what we lost when making this change I
left a comment, but there is no point to change the code to generate a
correct sequence at this point.

This is not a Fixes tagged patch on purpose because it doesn't change
the binary output.

Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>