libertas: fix format warning

drivers/net/wireless/libertas/if_spi.c: In function ‘if_spi_c2h_data’:
drivers/net/wireless/libertas/if_spi.c:733: warning: format ‘%u’ expects type ‘unsigned int’, but argument 4 has type ‘long unsigned int’

Signed-off-by: John W. Linville <linville@tuxdriver.com>