drivers: smbus: stm32: add cast to avoid warning

Previously, the function `smbus_stm32_pcall()` implicitly cast
`uint16_t *` to `uint8_t *`. Add an explicit cast to avoid warning.

```shell
..drivers/smbus/smbus_stm32.c:358:32: error: initialization of \
  'uint8_t *' {aka 'unsigned char *'} from incompatible pointer type \
  'uint16_t *' {aka 'short unsigned int *'} \
  [-Werror=incompatible-pointer-types]
  358 |                         .buf = &send_word,
      |                                ^
..drivers/smbus/smbus_stm32.c:358:32: note: (near initialization \
  for 'messages[1].buf')
..drivers/smbus/smbus_stm32.c:363:32: error: initialization of \
  'uint8_t *' {aka 'unsigned char *'} from incompatible pointer type \
  'uint16_t *' {aka 'short unsigned int *'} \
  [-Werror=incompatible-pointer-types]
  363 |                         .buf = recv_word,
      |                                ^~~~~~~~~
```

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

drivers/smbus/smbus_stm32.c

@@ -355,12 +355,12 @@ static int smbus_stm32_pcall(const struct device *dev, uint16_t periph_addr, uin
 			.flags = I2C_MSG_WRITE,
 		},
 		{
-			.buf = &send_word,
+			.buf = (uint8_t *)&send_word,
 			.len = sizeof(send_word),
 			.flags = I2C_MSG_WRITE,
 		},
 		{
-			.buf = recv_word,
+			.buf = (uint8_t *)recv_word,
 			.len = sizeof(*recv_word),
 			.flags = I2C_MSG_READ | I2C_MSG_RESTART,
 		},