gpio: wcove: Use IRQ hardware number getter instead of direct access

IRQ framework provides special type and getter to transform Linux IRQ
to the hardware pin. Use that type and getter function instead of
direct access. No functional changes intended.

While at it, remove unneeded check in wcove_update_irq_ctrl() since
it is guaranteed that function will be called with valid parameter.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>