Bluetooth: SPI: fix max SPI buffer length

Drop the BLUETOOTH_SPI_RX_BUFFER_SIZE and BLUETOOTH_SPI_TX_BUFFER_SIZE
config options by fixing the max SPI buffer length to 255, as used by
the X-NUCLEO-IDB04A1 BSP. This simplifies the rx/tx buffer handling, and
avoids a potential spi rx stack overflow depending on the config values
set by the user.

Change-Id: Ifa7fd086016abda4bdcf9638f28b38d001a288c5
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>