Fix ULSCH ID type to handle large max_nb_pusch values (ULSCH procedures)

The ULSCH_id variable is currently defined as uint8_t, which limits its
range to 0-255. However, gNB->max_nb_pusch can exceed this range
depending on the configuration (buffer_ul_slots and
MAX_MOBILES_PER_GNB).  This can lead to incorrect behavior or undefined
results when max_nb_pusch is larger than 255. This commit changes the
type of ULSCH_id from uint8_t to int to accommodate larger values of
max_nb_pusch.

The issue was observed when running the OAI gNB with MAX_MOBILES_PER_GNB
set to 64 UEs. The root cause was traced back to the changes to UL
processing introduced in !2952.