hdr.len includes both the size of the header and the fragment, so using
this when stepping through the firmware causes us to skip 16 bytes every
chunk of 3072 bytes; causing only the first fragment to actually be
valid data.

Instead use fragment size steps through the firmware blob.

Fixes: ea7a1f27 ("soc: qcom: Introduce WCNSS_CTRL SMD client")
Reported-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>