staging: wilc1000: remove unnecessary define FIVE_GHZ_BAND

wilc1000 driver does not support five Ghz band. Remove FIVE_GHZ_BAND and
it's related code.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>