bcmgenet: Add support for rgmii-rxid

The commit 57805f22 ("net: bcmgenet: Don't set ID_MODE_DIS when
not using RGMII") needed to be extended for the case of using the
rgmii-rxid.  The latest version of the Rasbperry Pi4 dtb files for the
5.4 now specify the rgmii-rxid.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Tested-by: Petr Tesarik <ptesarik@suse.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>