USB: android: Check ffs->state in ffs_ready_callback

In some cases race condition seen where adbd is being killed
and in parallel ffs_ready_callback() is executing.
ffs_ready_callback() checks config->enabled and tries to
re-enumerate selected USB composition. This USB composition
re-enumeration fails with error as:

ffs_function_bind_config(): usb_add_function() fails (err:-19) for ffs
android_bind_enabled_functions: ffs failed
android_enable: usb_add_config failed : err: -19
enable_store: enable_store: android_enable failed

Hence to avoid restarting of USB composition from ffs_ready_callaback()
if dev is NULL.

Change-Id: I92b7bf3e78ebcfe9195667229af67bc099ce0c9a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>