drivers: net: xgene: Fix module unload crash - clkrst sequence

This patch fixes clock reset sequence.

- Added clock reset sequence for ACPI
- Added delay in clock reset sequence to make sure pulse is generated
- Added clk_unprepare_disable() in port shutdown to make sure
  clock increment/decrement counts are matching
- Removed MII_MGMT_CONFIG programming, since it is not required
- Fixed programming XGENET_CONFIG_REG to enable SGMII mode

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>