lan743x: Provide Read/Write Access to on chip OTP

The LAN743x includes on chip One-Time-Programmable (OTP) memory.

This patch extends the ethtool EEPROM read/write interface to
access OTP memory space.

The currently existing interface is limited, as it does not
allow OTP read, and OTP writes are restricted to
offset==0, length==512, and data[0]==0xF3.

This patch removes these restrictions and adds a private flag
called OTP_ACCESS, which is used to switch between EEPROM, and
OTP modes.

The private flag OTP_ACCESS is configurable through the
  ethtool --set-priv-flags command.
And visible through the
  ethtool --show-priv-flags command.

By default OTP_ACCESS is false, and there for previously existing
EEPROM commands will work exactly the same. However now access to
OTP requires one extra step of setting OTP_ACCESS to true. This
flag controls the read, write, and length reporting, functions
of ethtool.

EEPROM presence is not checked when setting or clearing this flag.
If the EEPROM is not present, the user, as before, will need to
diagnose that using existing read and write function of ethtool,
while OTP_ACCESS is false.

Updates for V2:
Added comments as to why this patch is needed.
Added comments explaining that EEPROM presence is not check
  when setting or clearing the OTP_ACCESS flag.
Added length checking to all otp/eeprom read/write functions.

Signed-off-by: Bryan Whitehead <Bryan.Whitehead@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>