tty: serial: fsl_lpuart: use port struct directly to simply code

Most lpuart functions have the parameter struct uart_port *port, but
still use the &sport->port to get the uart_port instead of use it
directly, let's simply the code logic, directly use this struct instead
of covert it from struct sport.

Signed-off-by: Sherry Sun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: SherrySun5