al_eth: make function definitions consistent with declarations

[dim]

Summary

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.

MFC after: 3 days

[emaste]

OK, although we may want to either just use int to match struct al_eth_lm_retimer, or change struct al_eth_lm_retimer to al_bool as well.

[jhb]

Or just replace al_bool with C99 bool, but that can be done in a future change.