USB: serial: safe_serial: move __inline__ before return type

Make the code like the rest of the kernel.
Also use inline instead of __inline__.

Link: http://lkml.kernel.org/r/a5072b74b6c293e6ec93c4900482e9d3267f15b2.1499284835.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Johan Hovold <johan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>