net/ethernet: convert to use module_platform_driver in octeon_mgmt.c

Simplify the code by using module_platform_driver macro
for octeon_mgmt.

Signed-off-by: dingsenjie <dingsenjie@yulong.com>
Link: https://lore.kernel.org/r/20210128035330.17676-1-dingsenjie@163.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>