USB: serial: ftdi_sio: fix 300 bps rate for SIO

stable inclusion
from stable-v5.10.148
commit 3451df3a514cb97c6295f6c788511a6a62140709
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6D0WL

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3451df3a514cb97c6295f6c788511a6a62140709



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commit 7bd7ad3c upstream.

The 300 bps rate of SIO devices has been mapped to 9600 bps since
2003... Let's fix the regression.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>