UPSTREAM: wifi: nl80211: remove redundant err variable

Return value from rdev_set_mcast_rate() directly instead of
taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Bug: 253289327
Change-Id: I51b4422895e8ddd9522f8e2c4ea3c8131989f32c
(cherry picked from commit a21cd7d6)
Signed-off-by: Kiran Kumar Lokere <quic_klokere@quicinc.com>
Signed-off-by: Veerendranath Jakkam <quic_vjakkam@quicinc.com>