al_eth: make function definitions consistent with declarations

The declarations for al_eth_lm_retimer_ds25_signal_detect() and
al_eth_lm_retimer_ds25_cdr_lock() say that these functions return
'al_bool', but the definitions actually return 'boolean_t'.

Make the definitions match the declarations.

Reviewed by: jhb, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39759