powerpc/macintosh: Fix unsigned check in smu_sat_get_sdb_partition()

i2c_smbus_read_word_data() returns a s32, which may be negative
but unsigned len cannot be negative.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>