drivers: spi: silabs: eusart: Always enable peripheral

The EUSART peripheral must always be enabled in configure(), as
its enable state gets cleared when the state machine is unretained
in deep sleep. The rest of the config registers are retained,
so they can continue to not be repainted on every configure() if
the context is the same as last time.

Remove unnecessary duplicated enable at the end of configure(),
the SPIInit HAL function already performs enable.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>