drivers: net: smc911x: Work around set but unused status

drivers/net/ethernet/smsc/smc911x.c: In function ‘smc911x_phy_interrupt’:
drivers/net/ethernet/smsc/smc911x.c:976:6: warning: variable ‘status’ set but not used [-Wunused-but-set-variable]
  976 |  int status;

A comment indicates the status needs to be read from the PHY,
otherwise bad things happen. But due to the macro magic, it is hard to
perform the read without assigning it to a variable. So add
_always_unused attribute to status to tell the compiler we don't
expect to use the value.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>