USB: gadget: f_mass_storage: Fix unused variable warning

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>