if_spi: Fix compilation warning - unused variable 'spi'

drivers/net/wireless/libertas/if_spi.c: In function ‘if_spi_init_card’:
drivers/net/wireless/libertas/if_spi.c:1035: warning: unused variable ‘spi’

The variable is used only in a macro which uses the args only for DEBUG builds.
Remove the 'spi' variable completely.
Signed-off-by: Nikanth Karthikesan <nikanth@google.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>