net: dsa: b53: do not touch DLL_IQQD on bcm53115

net: dsa: b53: do not touch DLL_IQQD on bcm53115

According to OpenMDK, bit 2 of the RGMII register has a different
meaning for BCM53115 [1]:

"DLL_IQQD         1: In the IDDQ mode, power is down0: Normal function
                  mode"

Configuring RGMII delay works without setting this bit, so let's keep it
at the default. For other chips, we always set it, so not clearing it
is not an issue.

One would assume BCM53118 works the same, but OpenMDK is not quite sure
what this bit actually means [2]:

"BYPASS_IMP_2NS_DEL #1: In the IDDQ mode, power is down#0: Normal
                    function mode1: Bypass dll65_2ns_del IP0: Use
                    dll65_2ns_del IP"

So lets keep setting it for now.

[1] https://github.com/Broadcom-Network-Switching-Software/OpenMDK/blob/master/cdk/PKG/chip/bcm53115/bcm53115_a0_defs.h#L19871
[2] https://github.com/Broadcom-Network-Switching-Software/OpenMDK/blob/master/cdk/PKG/chip/bcm53118/bcm53118_a0_defs.h#L14392

Fixes: 967dd82ffc52 ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://patch.msgid.link/20250602193953.1010487-6-jonas.gorski@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>