CVE-2024-40967 serial: imx: Introduce timeout when waiting on transmitter empty

In the Linux kernel, the following vulnerability has been resolved:

serial: imx: Introduce timeout when waiting on transmitter empty

By waiting at most 1 second for USR2_TXDC to be set, we avoid a potential
deadlock.

In case of the timeout, there is not much we can do, so we simply ignore
the transmitter state and optimistically try to continue.