drivers: spi: gecko: usart: Implement lock

Implement the SPI context lock in the USART SPI driver for
EFM32/EFR32.

Remove incorrect -EBUSY error when spi_release() is called while
there are still bytes in the TX FIFO. This condition does not
cause the act of releasing the lock to fail.

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