USB: gadget: f_mass_storage: Fix unused variable warning

stable inclusion
from stable-v5.10.195
commit 44c71cbe7c5ac10390aef02ff2dd3965ba281660
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I95JOC

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=44c71cbe7c5ac10390aef02ff2dd3965ba281660



--------------------------------

[ Upstream commit 55c3e571 ]

Fix a "variable set but not used" warning in f_mass_storage.c.  rc is
used if	verbose debugging is enabled but not otherwise.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: d5e2b67a ("USB: g_mass_storage: template f_mass_storage.c file created")
Link: https://lore.kernel.org/r/cfed16c7-aa46-494b-ba84-b0e0dc99be3a@rowland.harvard.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>