mt76: mt7615: Fix build with older compilers

mt76: mt7615: Fix build with older compilers

Some compilers (tested with 4.8.5 from CentOS 7) fail properly process
FIELD_GET inside an inline function, which ends up in a BUILD_BUG_ON.
Convert inline function to a macro.

Fixes commit c45085ce7cf8 ("mt76: mt7615: add support for per-chain
signal strength reporting")
Reported in https://lkml.org/lkml/2019/9/21/146

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pablo Greco <pgreco@centosproject.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>