target/avr: Fix compiler errors (-Werror=enum-conversion)

../target/avr/translate.c: In function ‘gen_jmp_ez’:
../target/avr/translate.c:1012:22: error: implicit conversion from ‘enum <anonymous>’ to ‘DisasJumpType’ [-Werror=enum-conversion]
 1012 |     ctx->base.is_jmp = DISAS_LOOKUP;
      |                      ^

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Michael Rolnik <mrolnik@gmail.com>
Message-Id: <20210706180936.249912-1-sw@weilnetz.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>