Fixed initializing interrupts for direct GPIO when
CONFIG_ZMK_KSCAN_DIRECT_POLLING is not enabled. IS_ENABLED() is needed to map
the possibly-undefined value to 0 or 1 so COND_CODE_0() and COND_CODE_1() work.
- Add ZMK_KSCAN_MATRIX_POLLING config flag to Kconfig
- Update matrix driver code to use the above flag to conditionally add the handling code for polling operations.