Staging: iio: Change data type to u16 to avoid unnecessary typecast

In the adis16220_read16bit() function we earlier used a s16 value 'val'
which is used by the adis_read_reg_16 function to read data and takes a
u16 value as a parameter.

So, this patch changes the data type of 'val' from s16 to u16. It is safe
to remove the extra sign extension, since the user of the function uses it
to read a 10 unsigned value which will lead to the same result in both cases.
Further this patch removes the unnecessary typecast for the simplification of
code. In addition to this, initialization of 'val' to 0 is also dropped. This is
due to the fact that not initializing helps the compiler provide useful warnings
if the code gets changed to return an otherwise uninitialized result.

Signed-off-by: Somya Anand <somyaanand214@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>