flash/nor/jtagspi: 'retval' may be used uninitialized

flash/nor/jtagspi: 'retval' may be used uninitialized

Problem
As my compiler has "warnings being treated as errors" on, it shows the
error message:
  error: 'retval' may be used uninitialized in this function

Investigation
Nothing wrong with the logic, 'retval' would have a value before returning.
Just wanna get rid of the compiling "warning as error" message.

Solution
Provide a reasonable default value

Change-Id: I712c15f82819c6c48bee9dceca8de4b18aeb29b0
Signed-off-by: Alamy Liu <alamy.liu@gmail.com>
Reviewed-on: http://openocd.zylin.com/2905
Tested-by: jenkins
Reviewed-by: Robert Jordens <jordens@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>