Staging: rtl8712: use kmalloc_array over kmalloc

This patch fixes checkpatch.pl warning in file rtl871x_xmit.c
WARNING : prefer kmalloc_array over kmalloc with multiply

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>