Implement register write operation according to the control communication
specified in the OPEN Alliance 10BASE-T1x MACPHY Serial Interface
document. Control write commands are used by the SPI host to write
registers within the MAC-PHY. Each control write commands are composed of
a 32 bits control command header followed by register write data.

The MAC-PHY ignores the final 32 bits of data from the SPI host at the
end of the control write command. The write command and data is also
echoed from the MAC-PHY back to the SPI host to enable the SPI host to
identify which register write failed in the case of any bus errors.
Control write commands can write either a single register or multiple
consecutive registers. When multiple consecutive registers are written,
the address is automatically post-incremented by the MAC-PHY. Writing to
any unimplemented or undefined registers shall be ignored and yield no
effect.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Parthiban Veerasooran <Parthiban.Veerasooran@microchip.com>
Link: https://patch.msgid.link/20240909082514.262942-3-Parthiban.Veerasooran@microchip.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>