atl1c: clear PCIE error status in atl1c_reset_pcie

clear PCIE error status (error log is write-1-clear).
REG_PCIE_UC_SEVERITY is removed as it's a standard pcie register,
and using kernle API to access it.

Signed-off-by: xiong <xiong@qca.qualcomm.com>
Tested-by: Liu David <dwliu@qca.qualcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>