spi: spi-pci1xxxx: Fix Probe failure with Dual SPI instance with INTx interrupts

spi: spi-pci1xxxx: Fix Probe failure with Dual SPI instance with INTx interrupts

Fixes a probe failure that occurs when dual SPI controllers are
enabled and INTx interrupts are used. Reduces the minimum required
number of interrupt vectors to 1 and registers a shared ISR when
the allocated vectors are fewer than the number of controllers.
This change ensures that the probe succeeds even with limited
vectors, restoring INTx functionality when multiple SPI
controllers are present.

Signed-off-by: Thangaraj Samynathan <thangaraj.s@microchip.com>
Link: https://patch.msgid.link/20250527103244.26861-1-thangaraj.s@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>