serial: pl011: Do not clear RX FIFO & RX interrupt in unthrottle.

stable inclusion
from stable-v4.19.270
commit 3a25c7891d717db137354476e0bb6eb34ad5f2d3
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6DPF8


CVE: NA

--------------------------------

[ Upstream commit 032d5a71 ]

Clearing the RX FIFO will cause data loss.
Copy the pl011_enabl_interrupts implementation, and remove the clear
interrupt and FIFO part of the code.

Fixes: 211565b1 ("serial: pl011: UPSTAT_AUTORTS requires .throttle/unthrottle")
Signed-off-by: delisun <delisun@pateo.com.cn>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221110020108.7700-1-delisun@pateo.com.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>