Support for setting the RGMII_IDMODE bit was added in the commit
referenced below. However, that commit did not add the symmetrical
clearing of the bit by way of setting it in "mask". Add it here.

Note that the documentation marks clearing this bit as "reserved",
however, according to TI, support for delaying the clock does exist in
the MAC, although it is not officially supported.
We tested this on a board with an RGMII to RGMII link that will not
work unless this bit is cleared.

Fixes: 0fb26c30 ("drivers: net: cpsw-phy-sel: add support to configure rgmii internal delay")
Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Signed-off-by: David S. Miller <davem@davemloft.net>