staging: wilc1000: fix incorrect strncasecmp length

The strncasecmp of buff against the literal string RSSI
is using variable length which is zero. This should be instead
using the variable size instead.  Also remove the redundant
variable length.

Detected by PVS-Studio, warning: V575

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>