SH4: Fix swap.b

SH4: Fix swap.b

The SH4 manual documents the swap.b instruction as follows:

        SWAP.B Rm,Rn
                Rm → swap lower 2 bytes → Rn

Current QEMU code, in addition to the above, also clears the high
16 bits. The immediate breakage I saw is that htonl function applied
to  netmask of 255.255.255.0 gives 0, which breaks all networking.

(Vladimir Prus)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5471 c046a42c-6fe2-441c-8c8c-71466251a162