net: mvneta: fix section mismatch warning caused by mvneta_deinit()

mvneta_deinit() can be called from the ->probe() hook in the error
path, so it shouldn't be marked as __devexit. It fixes the following
section mismatch warning:

WARNING: vmlinux.o(.devinit.text+0x239c): Section mismatch in reference
from the function mvneta_probe() to the function .devexit.text:mvneta_deinit()
The function __devinit mvneta_probe() references
a function __devexit mvneta_deinit().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>