staging: wilc1000: remove function pointer wlan_handle_rx_isr

This patch removes function pointer wlan_handle_rx_isr and just call
the function wilc_handle_isr.

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>