Staging: iio/accel: Changed data type in adis16220_write_16bit to u16

In the adis16220_write_16bit() function we used a long value to store
parsed data from the char* buffer buf.
The called function to actually write the data,
adis16220_spi_write_reg_16(), takes a u16 value as a parameter, so up
to now a value larger than u16 was silently ignored as it was only
truncated when passing the parameter.
Now this function will only accept values fitting into a u16.

Additionally the parsing function was changed to overcome the now
obsolete strict_strtol() function.

Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>