scripts: process_gperf: upgrade the asso_values type to unsigned short

The gperf tool automatically selects the optimal data type for the
asso_values table, depending on MAX_HASH_VALUE. However, there is
a corner case when the tables generated on different stages of the
build process have different data types, causing a link-time error.
Upgrade the data type for the table from unsigned char to unsigned
short to at least exclude this 8-bit to 16-bit transition. There is
another potential issue with the 16-bit to 32-bit transition, but
it seems not very likely to have 65k kernel objects anytime soon.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>