iio:accel:stk8312: drop local buffer

Drop the local buffer in stk8312_trigger_handler() and use data->buffer
instead for bulk reads.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>