i2c: mux: pca954x: Refactor pca954x_irq_handler()

Refactor pca954x_irq_handler() to:
  - use for_each_set_bit() macro
  - use IRQ_RETVAL() macro

Above change makes code easy to read and understand.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>