staging: iio: adis16240: Remove mutex_lock() and mutex_unlock() function call.

Remove mutex_lock() and mutex_unlock() function calls, as the
adis16240_spi_read_signed() function can be run parallel and safely
multiple times. Also remove the mutex.h header file and comment, which
are no longer required.

As indio_dev is declared and initialized in adis16240_spi_read_signed(),
again declaration to same type and initialization to same value is not
required, remove it from adis16240_read_12bit_signed().

Simplify the return logic, by merging assignment and return into a single
line.

Signed-off-by: Varsha Rao <rvarsha016@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>