rpmsg: qcom_smd: increase bounce buffer size

Bounce buffer size is 4K, which cause buffer overflow when
incoming packet size greater than 4k and packet is wrap around
in FIFO.

To avoid buffer overflow use bounce buffer size same as FIFO size.

Change-Id: Ia23adda1de2bd6613def844fc88a565587bcd82d
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>