greybus: fw-download: Create a macro for length of firmware file's name

Create a macro representing the length of the firmware file's name and
use that instead of using magic number in the code.

Suggested-by: Greg Kroah-Hartman <gregkh@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>