iwlegacy: 4965-rs: avoid null pointer dereference error

il4965_rs_initialize_lq checks to see if sta is null, however, before that
check il4965_rs_use_green dereferences sta when intializing use_green.
Avoid a potential null pointer dereference error by only calling
il4965_rs_use_green after we are sure sta is not null.

Smatch analysis:

drivers/net/wireless/iwlegacy/4965-rs.c:2160 il4965_rs_initialize_lq() warn:
  variable dereferenced before check 'sta' (see line 2155)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>