staging: et131x: fix make W=1 build warning

fix make W=1 warning:
drivers/staging/et131x/et131x.c: In function ‘et1310_setup_device_for_multicast’:
drivers/staging/et131x/et131x.c:1052:6: warning: variable ‘pm_csr’ set but not used [-Wunused-but-set-variable]
  u32 pm_csr;
      ^
drivers/staging/et131x/et131x.c: In function ‘et1310_setup_device_for_unicast’:
drivers/staging/et131x/et131x.c:1101:6: warning: variable ‘pm_csr’ set but not used [-Wunused-but-set-variable]
  u32 pm_csr;
      ^
drivers/staging/et131x/et131x.c: In function ‘et131x_isr_handler’:
drivers/staging/et131x/et131x.c:4006:8: warning: variable ‘pm_csr’ set but not used [-Wunused-but-set-variable]
    u32 pm_csr;
        ^