target-alpha: fix palcode mask for user pal calls

(Also 6 bits for unprivileged calls)

Signed-off-by: Tristan Gingold <gingold@adacore.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7033 c046a42c-6fe2-441c-8c8c-71466251a162