spi: ch341: fix out-of-bounds memory access in ch341_transfer_one

spi: ch341: fix out-of-bounds memory access in ch341_transfer_one

Discovered by Atuin - Automated Vulnerability Discovery Engine.

The 'len' variable is calculated as 'min(32, trans->len + 1)',
which includes the 1-byte command header.

When copying data from 'trans->tx_buf' to 'ch341->tx_buf + 1', using 'len'
as the length is incorrect because:

1. It causes an out-of-bounds read from 'trans->tx_buf' (which has size
   'trans->len', i.e., 'len - 1' in this context).
2. It can cause an out-of-bounds write to 'ch341->tx_buf' if 'len' is
   CH341_PACKET_LENGTH (32). Writing 32 bytes to ch341->tx_buf + 1
   overflows the buffer.

Fix this by copying 'len - 1' bytes.

Fixes: 8846739f52af ("spi: add ch341a usb2spi driver")
Signed-off-by: Tianchu Chen <flynnnchen@tencent.com>
Link: https://patch.msgid.link/20251128160630.0f922c45ec6084a46fb57099@linux.dev
Signed-off-by: Mark Brown <broonie@kernel.org>