net: e1000: fix indentation

Apparently the indentation is off here, for the IGB model just want to
bail out early.
Fix this to avoid both compiler warnings and puzzled readers.

Pointed out by GCC 6.2's -Wmisleading-indentation warning.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>