Test the incoming debug_level, rather than the current.

Test the incoming debug_level, rather than the current.

The existing code tests the current debug_level for validity, allowing
the user to set it too high and never change it after that.  The new
behaviour is to test the debug_level that the user has requested.

Change-Id: I85726a2e606c8d137e9b1cfe76fee865084844b1
Signed-off-by: Alan Bowman <alan.michael.bowman@gmail.com>
Reviewed-on: http://openocd.zylin.com/764
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>