Download client with SMP dfu over uart crash.

[marcusl97]

Hi, i have been working on getting dfu over uart with smp and mcumgr working on a nrf9160dk.

I kept getting a fatal error: ***** SECURE FAULT ***** Attribution unit violation error where the dumped adress only told me that it came from crc16_sw.c. After a lot of debugging i found out that when CONFIG_SMP_CMD_RETRY_TIME was set too short the download client would time out. Then since img_mgmt_client_upload_init starts a workthread that by default keeps trying to send the data via serial_util.c: mcumgr_serial_tx_pkt() for 15 seconds every CONFIG_SMP_CMD_RETRY_TIME milliseconds. But when the download client fails the data is gone and there isn't any cancellation of the smp transfer or any null check in mcumgr_serial_tx_pkt(). I have fixed it by increasing the CONFIG_SMP_CMD_RETRY_TIME to 1000 and adding a null check in mcumgr_serial_tx_pkt().