iwlegacy: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Enabling DEBUG_STRICT_USER_COPY_CHECKS causes the following
warning:

In file included from arch/x86/include/asm/uaccess.h:573,
                 from include/net/checksum.h:25,
                 from include/linux/skbuff.h:28,
                 from drivers/net/wireless/iwlegacy/iwl-4965-rs.c:28:
In function 'copy_from_user',
    inlined from 'iwl4965_rs_sta_dbgfs_scale_table_write' at
    drivers/net/wireless/iwlegacy/iwl-4965-rs.c:2616:
arch/x86/include/asm/uaccess_64.h:65:
warning: call to 'copy_from_user_overflow' declared with
attribute warning: copy_from_user() buffer size is not provably
correct

presumably due to buf_size being signed causing GCC to fail to
see that buf_size can't become negative.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>