staging: wilc1000: remove define RX_BH_TYPE

This patch removes the preprocessor definition, RX_BH_TYPE and uses the
request_threaded_irq instead of kernel thread to execute the interrupt
handler.

As a result, linux_wlan_enable_irq() and linux_wlan_disable_irq() are also
removed from the linux_wlan.c file.

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>