greybus: firmware/bootrom: debug output from bootrom protocol

The bootrom protocol issues no dynamic debugging messages when it functions
successfully.  Use dev_dbg() to fix that by issuing kernel logs when firmware
download works.

Signed-off-by: Eli Sennesh <esennesh@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>