iio: chemical: mhz19b: reject oversized serial replies

In the Linux kernel, the following vulnerability has been resolved: iio: chemical: mhz19b: reject oversized serial replies mhz19b_receive_buf() appends each serdev chunk into the fixed MHZ19B_CMD_SIZE receive buffer and advances buf_idx by len without checking that the chunk fits in the remaining space. A large callback can therefore overflow st->buf before the command path validates the reply. Reset the reply state before each command and reject oversized serial replies before copying them into the fixed buffer. When an oversized reply is detected, wake the waiter and report -EMSGSIZE instead of overwriting st->buf.