HID: intel_ish-hid: Add check for ishtp_dma_tx_map

As the kcalloc may return NULL pointer,
it should be better to check the ishtp_dma_tx_map
before use in order to avoid NULL pointer dereference.

Fixes: 3703f53b99e4 ("HID: intel_ish-hid: ISH Transport layer")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
