net: freescale: ucc_geth: Hardcode the preamble length to 7 bytes

net: freescale: ucc_geth: Hardcode the preamble length to 7 bytes

The preamble length can be configured in ucc_geth, however it just
ends-up always being configured to 7 bytes, as nothing ever changes the
default value of 7.

Make that value the default value when the MACCFG2 register gets
initialized, and remove the code to configure that value altogether.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>