iio: magnetometer: rm3100: add boundary check for the value read from RM3100_REG_TMRC

Recently, we encounter kernel crash in function rm3100_common_probe
caused by out of bound access of array rm3100_samp_rates (because of
underlying hardware failures). Add boundary check to prevent out of
bound access.

Fixes: 121354b2

 ("iio: magnetometer: Add driver support for PNI RM3100")
Suggested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: zhili.liu <zhili.liu@ucas.com.cn>
Link: https://lore.kernel.org/r/1704157631-3814-1-git-send-email-zhouzhouyi@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>