wcnss: fix integer underflow in wcnss_wlan

Fix integer underflow which may eventually results in an buffer
overread in wcnss_nvbin_dnld when the firmware file size is less
than 4 Byte. Add a check on file size before performing 
arithmetic operation which avoids buffer underflow.

CRs-Fixed: 2279226
Change-Id: Ia7fdb859e8c999f8a2e81c957c7cab35ef312844
Signed-off-by: Sandeep Singh <sandsing@codeaurora.org>