staging: wilc1000: rename wilc_firmware in the struct wilc

This patch renames wilc_firmware in the struct wilc to the firmware. In
addition, null assignments to the wl->firmware after release_firmware
are removed because it is not necessary.

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