drm/nouveau/bios/ramcfg: fix missing parentheses when calculating RON

Currently, the expression for calculating RON is always going to result
in zero no matter the value of ram->mr[1] because the ! operator has
higher precedence than the shift >> operator.  I believe the missing
parentheses around the expression before appying the ! operator will
result in the desired result.

[ Note, not tested ]

Detected by CoveritScan, CID#1324005 ("Operands don't affect result")

Fixes: c25bf7b6 ("drm/nouveau/bios/ramcfg: Separate out RON pull value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>