libertas: remove set but not used variable 'int_type'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/marvell/libertas/if_spi.c: In function 'if_spi_h2c':
drivers/net/wireless/marvell/libertas/if_spi.c:799:6: warning:
 variable 'int_type' set but not used [-Wunused-but-set-variable]

It never used since introduction in
commit d2b21f19 ("libertas: if_spi, driver for libertas GSPI devices")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Tested-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>