usb: mdm_data: Add support for QDLOADER bridging

Add necessary support to flash images to connected MDM in EDL
mode from host PC, via external AP. Also, add a module parameter
'prevent_edl_probe' to prevent 9008 PID (EDL PID) to bind to
ks_bridge driver.

Usage:
  To prevent binding to ks_bridge and bind to mdm_data_bridge:
    echo 1 > /sys/module/ks_bridge/parameters/prevent_edl_probe

  To switch back to default behavior (allow bind to ks_bridge):
    echo 0 > /sys/module/ks_bridge/parameters/prevent_edl_probe

While at it, fix potential null pointer dereference in
data_bridge_write.

Change-Id: I170c820803f016a03dcc5952424358e1a9835cfa
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>