tty: serial: fsl_lpuart: use kzalloc() instead of kmalloc()

Use kzalloc() instead of kmalloc() to get clean rx buffer
that is useful for DMA mode debug to check the data moving
validity.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20190717051930.15514-4-fugang.duan@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>