target-alpha: Fix compiler warning for gcc-4.3 (and older)

"Old" compilers obviously are not able to recognise
that all cases are handled here:

qemu/target-alpha/helper.c:70: error: ‘round_mode’ may be used uninitialized in this function

A small modification helps the compiler to do its jobs.

gcc-4.4 does not need this, but is still not standard on all platforms.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
