alpha: fix lit sign

alpha: fix lit sign

according to the alpha arch reference, the literal field of an operate
instruction is unsigned:

If bit <12> of the instruction is 1, an 8-bit zero-extended literal
constant is formed by bits
<20:13> of the instruction. The l teral is interpreted as a positive
integer bet ween 0 and 255
and is zero-extended to 64 bits.

This patch fixes the mis-interpretation of the literal field.

(Tristan Gingold)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5211 c046a42c-6fe2-441c-8c8c-71466251a162