nl80211: improve nl80211_parse_mesh_config type checking

When building a kernel with W=1, the nl80211.c file causes a number of
warnings, all about the same problem:

net/wireless/nl80211.c: In function 'nl80211_parse_mesh_config':
net/wireless/nl80211.c:5287:103: error: comparison is always false due to limited range of data type [-Werror=type-limits]
net/wireless/nl80211.c:5290:96: error: comparison is always false due to limited range of data type [-Werror=type-limits]
net/wireless/nl80211.c:5293:124: error: comparison is always false due to limited range of data type [-Werror=type-limits]
net/wireless/nl80211.c:5295:148: error: comparison is always false due to limited range of data type [-Werror=type-limits]
net/wireless/nl80211.c:5298:106: error: comparison is always false due to limited range of data type [-Werror=type-limits]
net/wireless/nl80211.c:5305:116: error: comparison is always false due to limited range of data type [-Werror=type-limits]

The problem is that gcc does not notice that the check is generate
by a macro, so it complains about comparing an unsigned type against 0.

I've tried to come up with a way to rephrase that code in a way that
avoids the warnings and otherwise improves the code as well.

This uses a set of new helper functions that perform the range checking,
and should provide slightly better type safety than the older patch,
at the expense of adding 44 lines to the code. Binary code size is
basically unchanged though (20 bytes added to 126561 bytes .text).

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>