cnss2: re-arch cnss_wlfw_bdf_dnld_send_sync

Re-arch cnss_wlfw_bdf_dnld_send_sync to reduce code complexity.
Create a new function to get BDF file name. This function will
be called later by cnss_wlfw_bdf_dnld_send_sync.

CRs-Fixed: 2510015
Change-Id: I2b6c0e34060c985c07790075796a103e57520ff8
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>