drivers/net/ftgmac100: refactor ftgmac100_reset_task to enable direct function call

This is to prepare for ftgmac100_adjust_link() to call reset function
directly, instead of task schedule.

Signed-off-by: Heyi Guo <guoheyi@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>