sc16is7xx: move label 'err_spi' to correct section

err_spi is used when SERIAL_SC16IS7XX_SPI is enabled, so make
the label only available under SERIAL_SC16IS7XX_SPI option.
Otherwise, the below warning appears.

drivers/tty/serial/sc16is7xx.c:1523:1: warning: label ‘err_spi’ defined but not used [-Wunused-label]
 err_spi:
  ^~~~~~~

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Fixes: ac0cdb3d ("sc16is7xx: missing unregister/delete driver on error in sc16is7xx_init()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>