olpc_battery: Fix endian neutral breakage for s16 values

When the driver was updated to be endian neutral (8e9c7716c)
the signed part of the s16 values was lost.  This is because be16_to_cpu()
returns an unsigned value.  This patch casts the values back to a s16
number prior to the the implicit cast up to an int.

Signed-off-by: Richard A. Smith <richard@laptop.org>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Cc: <stable@kernel.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>