net: e1000: Remove unused bus_to_phys() macro

bus_to_phys() is defined but not referenced at all. This patch removes
it completely.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>