atmel: remove set but not used variable 'dev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/atmel/atmel_cs.c:120:21:
 warning: variable dev set but not used [-Wunused-but-set-variable]

It is never used, so can remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>