tty: serial: Remove call to memset after pci_alloc_consistent

pci_alloc_consistent calls dma_alloc_coherent directly.
In commit 518a2f19
("dma-mapping: zero memory returned from dma_alloc_*"),
dma_alloc_coherent has already zeroed the memory.
So memset is not needed.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Link: https://lore.kernel.org/r/20190715032001.7212-1-huangfq.daxian@gmail.com


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