drivers: charger: charger_max20335: drop redundant validation

`max20335_get_constant_charge_current()` and
`max20335_set_chgin_to_sys_current_limit()` functions has redundant input
values validation. Drop not important ones to make the code clean.
While on it remove `max20335_get_constant_charge_voltage()` function
since it become useless.

Signed-off-by: Lukasz Madej <l.madej@grinn-global.com>