ppc: avoid write only variables

ppc: avoid write only variables

Compiling with GCC 4.6.0 20100925 produced warnings:
/src/qemu/target-ppc/op_helper.c: In function 'helper_icbi':
/src/qemu/target-ppc/op_helper.c:351:14: error: variable 'tmp' set but not used [-Werror=unused-but-set-variable]
/src/qemu/target-ppc/op_helper.c: In function 'do_6xx_tlb':
/src/qemu/target-ppc/op_helper.c:3805:28: error: variable 'EPN' set but not used [-Werror=unused-but-set-variable]
/src/qemu/target-ppc/op_helper.c: In function 'do_74xx_tlb':
/src/qemu/target-ppc/op_helper.c:3838:28: error: variable 'EPN' set but not used [-Werror=unused-but-set-variable]

Fix by adding a dummy cast so that the variable is not unused. Delete tmp.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>