riscv: fix minor typo in syscall.h comment

Some developers may be confused because RISC-V does not have
a register named r0. Also, orig_r0 is not available in pt_regs structure,
which is specific to riscv. So we had better fix this minor typo.

Signed-off-by: Austin Kim <austin.kim@lge.com>
Link: https://patch.msgid.link/aW3Z4zTBvGJpk7a7@adminpc-PowerEdge-R7525
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: Austin Kim

arch/riscv/include/asm/syscall.h

@@ -20,7 +20,7 @@ extern void * const sys_call_table[];
 extern void * const compat_sys_call_table[];
 
 /*
- * Only the low 32 bits of orig_r0 are meaningful, so we return int.
+ * Only the low 32 bits of orig_a0 are meaningful, so we return int.
  * This importantly ignores the high bits on 64-bit, so comparisons
  * sign-extend the low 32 bits.
  */