drivers: spi_nrfx: Fix NRFX_ASSERT crash when sck pin is set to no-connect

Fixes a bug in nrfx spi when used in modes where only MOSI is needed such
as driving a WS2812. In this mode NRFX_ASSERT() is triggered when sck pin
is configured as no-connect.

Signed-off-by: Anthony Williams <anthony289478@gmail.com>