remoteproc: avoid stack overflow in debugfs file

Recent gcc versions warn about reading from a negative offset of
an on-stack array:

drivers/remoteproc/remoteproc_debugfs.c: In function 'rproc_recovery_write':
drivers/remoteproc/remoteproc_debugfs.c:167:9: warning: 'buf[4294967295u]' may be used uninitialized in this function [-Wmaybe-uninitialized]

I don't see anything in sys_write() that prevents us from
being called with a zero 'count' argument, so we should
add an extra check in rproc_recovery_write() to prevent the
access and avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 2e37abb8 ("remoteproc: create a 'recovery' debugfs entry")
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>