[RISCV] Use bits<7> instead of bits<11> for the EEW field size in the...

[RISCV] Use bits<7> instead of bits<11> for the EEW field size in the RISCVZvlsseg searchable table. NFCI

We only support 8, 16, 32, and 64 for EEW. These only need 7 bits
to represent.