drivers: serial: ra_sci: revise switch-case in `callback_adapter`

Unified switch-case usage in `uart_ra_sci_callback_adapter` to use
`break` instead of `return`.
Typically, a `break` is used in switch-case statements unless an early
return is necessary, in which case `return` is appropriate.

For this case, using a `break` statement is the more suitable choice.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>