ramips: mt7620: handle request_irq fail in gsw ethernet driver

ramips: mt7620: handle request_irq fail in gsw ethernet driver

Handle return from request_irq in gsw ethernet driver.
Fix compilation warning:
drivers/net/ethernet/ralink/gsw_mt7620.c: In function 'mtk_gsw_init':
drivers/net/ethernet/ralink/gsw_mt7620.c:236:17: error: ignoring return value of 'request_irq' declared with attribute 'warn_unused_result' [-Werror=unused-result]
  236 |                 request_irq(gsw->irq, gsw_interrupt_mt7620, 0,
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  237 |                             "gsw", priv);
      |                             ~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>