avr32: fix relocation check for signed 18-bit offset

commit e68c636d upstream.

Caught by static code analysis by David.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>