hwmon: (ibmaem) Avoid repeated memory allocations

Preallocate a buffer for the response to sensor reads, and reuse it
for each read instead of allocating a new one each time. This should
be faster and should also avoid memory fragmentation.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Darrick J. Wong <djwong@us.ibm.com>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>