olpc_battery: Fix endian neutral breakage for s16 values

commit 7cfbb29 upstream.

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>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>