net/mac8390: discard unnecessary breaks

The 'break' is unnecessary because of previous 'return',
and we could discard it.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Link: https://lore.kernel.org/r/20201027135159.71444-1-zhangqilong3@huawei.com


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