net: mvneta: fix usage as a module on RGMII configurations

Commit 5445eaf3

 ('mvneta: Try to fix mvneta when compiled as
module') fixed the mvneta driver to make it work properly when loaded
as a module in SGMII configuration, which was tested successful by the
author on the Armada XP OpenBlocks AX3, which uses SGMII.

However, it turns out that the Armada XP GP, which uses RGMII, is
affected by a similar problem: its SERDES configuration is lost when
mvneta is loaded as a module, because this configuration is set by the
bootloader, and then lost because the clock is gated by the clock
framework until the mvneta driver is loaded again and the clock is
re-enabled.

However, it turns out that for the RGMII case, setting the SERDES
configuration is not sufficient: the PCS enable bit in the
MVNETA_GMAC_CTRL_2 register must also be set, like in the SGMII
configuration.

Therefore, this commit reworks the SGMII/RGMII initialization: the
only difference between the two now is a different SERDES
configuration, all the rest is identical.

In detail, to achieve this, the commit:

 * Renames MVNETA_SGMII_SERDES_CFG to MVNETA_SERDES_CFG because it is
   not specific to SGMII, but also used on RGMII configurations.

 * Adds a MVNETA_RGMII_SERDES_PROTO definition, that must be used as
   the MVNETA_SERDES_CFG value in RGMII configurations.

 * Removes the mvneta_gmac_rgmii_set() and mvneta_port_sgmii_config()
   functions, and instead directly do the SGMII/RGMII configuration in
   mvneta_port_up(), from where those functions where called. It is
   worth mentioning that mvneta_gmac_rgmii_set() had an 'enable'
   parameter that was always passed as '1', so it was pretty useless.

 * Reworks the mvneta_port_up() function to set the MVNETA_SERDES_CFG
   register to the appropriate value depending on the RGMII vs. SGMII
   configuration. It also unconditionally set the PCS_ENABLE bit (was
   already done for SGMII, but is now also needed for RGMII), and sets
   the PORT_RGMII bit (which was already done for both SGMII and
   RGMII).

This commit was successfully tested with mvneta compiled as a module,
on both the OpenBlocks AX3 (SGMII configuration) and the Armada XP GP
(RGMII configuration).

Reported-by: Steve McIntyre <steve@einval.com>
Cc: stable@vger.kernel.org # 3.11.x: 5445eaf3

 mvneta: Try to fix mvneta when compiled as module
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>