rtlwifi: rtl8723: Add error handling to no existing firmware

Without firmware, driver wastes time to download and wait for MCU bootup,
and then kernel core dump finally. If request_firmware fails, the value
max_fw_size=0 is set, so we check the value before downloading firmware.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>