mwifiex: fix loop timeout in mwifiex_prog_fw_w_helper()

USB8XXX_FW_MAX_RETRY is 3.  We were using a post-op loop
"while (retries--) {" but then the lines after that assume the loop
exits with retries set to zero.

I've fixed this by changing to a pre-op loop.  I started with retries
set to 4 instead of 3 so that we still go through the loop the same
number of times.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>