usb: musb: Fix NULL check on struct musb_request field

usb: musb: Fix NULL check on struct musb_request field

The 'request' variable is a pointer to the 'request' field of the
struct musb_request 'req' pointer. It only worked until now because
the 'request' field is the first one in the musb_request structure, but
as soon as that changes, the check will be invalid.

Fix it preventively by doing the NULL-check on the 'req' pointer
instead.

Suggested-by: Maarten ter Huurne <maarten@treewalker.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20210123142502.16980-2-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>