powerpc/mpic_msgr: fix cast removes address space of expression warnings

When build Linux kernel, encounter the following warnings:

./arch/powerpc/sysdev/mpic_msgr.c:230:38: warning: cast removes address space '__iomem' of expression
./arch/powerpc/sysdev/mpic_msgr.c:230:27: warning: incorrect type in assignment (different address spaces)

The data type of msgr->mer and msgr->base are 'u32 __iomem *', but
converted to 'u32 *' and 'u8 *' directly and cause above warnings, now
instead of using a type cast, change the size of the pointer offset to fix
these warnings.

Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221019063414.3758087-1-ruanjinjie@huawei.com

arch/powerpc/sysdev/mpic_msgr.c

@@ -20,7 +20,7 @@
 
 #define MPIC_MSGR_REGISTERS_PER_BLOCK	4
 #define MPIC_MSGR_STRIDE		0x10
-#define MPIC_MSGR_MER_OFFSET		0x100
+#define MPIC_MSGR_MER_OFFSET		(0x100 / sizeof(u32))
 #define MSGR_INUSE			0
 #define MSGR_FREE			1
 
@@ -234,7 +234,7 @@ static int mpic_msgr_probe(struct platform_device *dev)
 
 		reg_number = block_number * MPIC_MSGR_REGISTERS_PER_BLOCK + i;
 		msgr->base = msgr_block_addr + i * MPIC_MSGR_STRIDE;
-		msgr->mer = (u32 *)((u8 *)msgr->base + MPIC_MSGR_MER_OFFSET);
+		msgr->mer = msgr->base + MPIC_MSGR_MER_OFFSET;
 		msgr->in_use = MSGR_FREE;
 		msgr->num = i;
 		raw_spin_lock_init(&msgr->lock);