mt76: mt7615: Fix build with older compilers

commit f53300fd upstream.

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 bf92e768 ("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>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>