boards: arduino_101: Fix separated-line interrupt Kconfig setting

The SPI_DW_INTERRUPT_SEPARATED_LINES symbol was removed and replaced by
SPI_DW_PORT_{0,1,2,3}_INTERRUPT_SINGLE_LINE in commit 423f0095
("drivers/spi: Specify options per-port on DW driver"), but an
assignment to it remains.

Remove the assignment and set SPI_DW_PORT_0_INTERRUPT_SINGLE_LINE to 'n'
instead. The Arduino has a single SPI port.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>