usb: gadget: composite: Add spinlock protection for usb string descriptor

During composition switch, android driver stops data transfers first
and removes configuration before disabling the pullup. With this sequence
there is a possibility for a race where usb_remove_config is in progress
during which pullup is active and sending the setup request for strings
with zero configuration value. Hence fix the issue by adding spinlock
protection for get_sring descriptor.

Change-Id: I13c601f0e48d847b322a2761cd52268963cacf01
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>