std_init(): Drop check if pass in driver is non-NULL

std_init(): Drop check if pass in driver is non-NULL

std_init() checks if the pass in struct sr_dev_driver is non-NULL and
prints a error message and returns an error if it is NULL.

std_init() is exclusively called from driver init() callbacks for which the
core already checks if the struct sr_dev_driver is non-NULL before invoking
the callback. This means the check in std_init() will always evaluate to
false. So drop this check.

This also means that the prefix parameter that was used in the error
message is no longer needed and can be removed from the function signature.
Doing so will make the std_init() function signature identical to the
init() callback signature which will allow to directly use it as such.

Signed-off-by: Lars-Peter Clausen <redacted>