tty: serial: fsl_lpuart: Fix unused variable 'sport' build warning

tty: serial: fsl_lpuart: Fix unused variable 'sport' build warning

Remove the unused variable 'sport' to avoid the kernel build warning.

Fixes: 3cc16ae096f1 ("tty: serial: fsl_lpuart: use port struct directly to simply code")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503210614.2qGlnbIq-lkp@intel.com/
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20250324021051.162676-1-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>