ath10k: fix backtrace on coredump

In a multiradio board with one QCA9984 and one AR9987
after enabling the crashdump with module parameter
coredump_mask=7, below backtrace is seen.

vmalloc: allocation failure: 0 bytes
 kworker/u4:0: page allocation failure: order:0, mode:0x80d2
 CPU: 0 PID: 6 Comm: kworker/u4:0 Not tainted 3.14.77 #130
 Workqueue: ath10k_wq ath10k_core_register_work [ath10k_core]
 (unwind_backtrace) from [<c021abf8>] (show_stack+0x10/0x14)
 (dump_stack+0x80/0xa0)
 (warn_alloc_failed+0xd0/0xfc)
 (__vmalloc_node_range+0x1b4/0x1d8)
 (__vmalloc_node+0x34/0x40)
 (vzalloc+0x24/0x30)
 (ath10k_coredump_register+0x6c/0x88 [ath10k_core])
 (ath10k_core_register_work+0x350/0xb34 [ath10k_core])
 (process_one_work+0x20c/0x32c)
 (worker_thread+0x228/0x360)

This is due to ath10k_hw_mem_layout is not defined for AR9987.
For coredump undefined hw ramdump_size is 0.
Check for the ramdump_size before allocation memory.

Tested on: AR9987, QCA9984
FW version: 10.4-3.9.0.2-00044

Signed-off-by: Anilkumar Kolli <akolli@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>