iio: imu: st_lsm6dsx: pre-allocate read buffer at bootstrap

Allocate device read buffer at bootstrap and do not put it on the stack
since it is pretty big (~200B) and its size will increase adding support
to device hw timestamp.
Moreover this patch fixes following sparse warnings:
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:250:17: warning: Variable length
array is used.
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:283:55: error: cannot size
expression

Fixes: 290a6ce1 ("iio: imu: add support to lsm6dsx driver")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>