staging: iio: adis16209: Remove mutex_lock() and mutex_unlock() calls

The function adis16209_read_raw() is safe to be run in parallel.
The call to adis_read_reg_16() is safe since adis_read_reg() uses the
txrx_lock from struct adis to protect simultaneous changes.

Remove mutex.h inclusion since it is no longer needed.

Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>