Fix warnings

hw/mips_r4k.c: In function  ‘mips_r4kc_init’:
hw/mips_r4k.c:230: warning: pointer targets in passing
argument 3 of ‘load_elf’ differ in signedness
hw/mips_r4k.c:256: warning: pointer targets in passing
argument 1 of ‘strcpy’ differ in signedness