f_gsi: Use kernel buffer instead of user space provided buffer

gsi_ctrl_dev_write() and gsi_ctrl_dev_read() API is directly using user
space provided buffer when qti_packet_debug is enabled. This can result
into using untrusted buffer pointer. Hence use cpkt->buf i.e. kernel
space buffer pointer instead of directly using user space provided buffer.

CRs-Fixed: 2061391
Change-Id: Iba6f2845dae2755446b4b8e9f3041686877d7bc4
Signed-off-by: Mayank Rana <mrana@codeaurora.org>