stm32: do not use private constructor

[hvraven]

I see compile errors using a recent git checkout of embassy with an stm32l431cb. Use the from_bits function instead.

error[E0603]: tuple struct constructor `Sloten` is private
    --> /usr/local/cargo/git/checkouts/embassy-9312dcb0ed774b29/17301c0/embassy-stm32/src/sai/mod.rs:939:36
     |
939  |                 w.set_sloten(vals::Sloten(config.slot_enable as u16));
     |                                    ^^^^^^ private tuple struct constructor
     |
    ::: /usr/local/cargo/git/checkouts/stm32-data-generated-cb34dad5f3150296/5ffbb82/stm32-metapac/src/chips/stm32l431cb/../../peripherals/sai_v2.rs:1523:23
     |
1523 |     pub struct Sloten(u16);
     |                       --- a constructor is private if any of the fields is private
     |
note: the tuple struct constructor `Sloten` is defined here
    --> /usr/local/cargo/git/checkouts/stm32-data-generated-cb34dad5f3150296/5ffbb82/stm32-metapac/src/chips/stm32l431cb/../../peripherals/sai_v2.rs:1523:5
     |
1523 |     pub struct Sloten(u16);
     |     ^^^^^^^^^^^^^^^^^