drivers: spi: litex: litespi: flush pending irq before enable

flush pending irq before enableing the irq.

In the litex soc, if an irq is not enabled, it will
it can still be registered and will be pending
if it occurs, it will just not be send to the cpu
until the irq is enabled.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>