regdb: Generalize the mW to dBm power conversion

Generalize the power conversion from mW to dBm
using log. This should fix the below compilation
error for country NO which adds a new power value
2000mW which is not handled earlier.

 CC [M]  net/wireless/wext-sme.o
 CC [M]  net/wireless/regdb.o
net/wireless/regdb.c:1130:1: error: Unknown undeclared here (not in
a function)
net/wireless/regdb.c:1130:9: error: expected } before power
make[2]: *** [net/wireless/regdb.o] Error 1
make[1]: *** [net/wireless] Error 2
make: *** [net] Error 2

Reported-By: John Walker <john@x109.net>
Signed-off-by: Chaitanya T K <chaitanya.mgit@gmail.com>
Acked-by: John W. Linville <linville@tuxdriver.com>
[remove unneeded parentheses, fix rounding by using %.0f]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>