soc: qcom: wcnss_ctrl: Fix increment in NV upload

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>