program: Use SOL referral fee for SOL portion of deposit during deposit-stake

[joncinque]

Problem

The process_deposit_stake function in spl-stake-pool applies calc_pool_tokens_stake_referral_fee to the combined total of both stake and SOL deposit fees.

Proposed Solution

Maybe the SOL fee component should use calc_pool_tokens_sol_referral_fee instead.

The current behavior was done for simplicity, but perhaps it should be revisited