wl1271: refactor debugfs function generation code

refactor wl1271_debugfs by using a format&copy function, instead of
duplicating the code for each generated function.

this change reduces about 3Kb from wl1271.ko
Signed-off-by: Eliad Peller <eliad@wizery.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>