iio:st_sensors: Fix oops when probing SPI devices

iio:st_sensors: Fix oops when probing SPI devices

commit 8e71c04f863a1754f21b27fb8ecb773d680a0a80 upstream.

In SPI mode the transfer buffer is locked with a mutex. However this
mutex is only initilized after the probe, but some transfer needs to
be done in the probe.

To fix this bug we move the mutex initialization at the beginning of
the device probe.

Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>