param: lock the bus as short as possible (#5187)

Since the FRAM and the baro are on the same bus on the Pixracer, we
currently need to lock down everything (instead of just this SPI bus)
for the time when the params are written.
Therefore, we need to keep this locking as short as possible.

This change makes the locking even shorter by moving all param_get and
param_name and param_size calls out of the lock.