staging: iio: adis16240: fix sparse warnings regarding incorrect argument type

Silence the following sparse warnings by changing cast from u16 to
__be16:

  CHECK   drivers/staging/iio/accel/adis16240_core.c
drivers/staging/iio/accel/adis16240_core.c:128:51: warning: incorrect
type in argument 3 (different signedness)
drivers/staging/iio/accel/adis16240_core.c:128:51:    expected unsigned
short [usertype] *val
drivers/staging/iio/accel/adis16240_core.c:128:51:    got signed short
*<noident>
drivers/staging/iio/accel/adis16240_core.c:142:51: warning: incorrect
type in argument 3 (different signedness)
drivers/staging/iio/accel/adis16240_core.c:142:51:    expected unsigned
short [usertype] *val
drivers/staging/iio/accel/adis16240_core.c:142:51:    got signed short
*<noident>

Signed-off-by: Teodora Baluta <teodora.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>