staging: iio: ad5933: fix format string warnings

Fixed the following warnings (reported by cppcheck):
[drivers/staging/iio/impedance-analyzer/ad5933.c:363]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/impedance-analyzer/ad5933.c:367]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/impedance-analyzer/ad5933.c:367]: (warning) %d in format string (no. 2)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/impedance-analyzer/ad5933.c:367]: (warning) %d in format string (no. 3)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/impedance-analyzer/ad5933.c:367]: (warning) %d in format string (no. 4)
requires 'int' but the argument type is 'unsigned int'.

Signed-off-by: Asaf Vertz <asaf.vertz@tandemg.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>