iio: adc: ti-ads7950: Fix improper use of mlock

[ Upstream commit abbde2792999c9ad3514dd25d7f8d9a96034fe16 ]

Indio->mlock is used for protecting the different iio device modes.
It is currently not being used in this way. Replace the lock with
an internal lock specifically used for protecting the SPI transfer
buffer.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>