Simplify the logic at ttusb_dec_send_command().

Besides avoiding some code duplication, as a side effect,
this could remove this false positive return with spatch:

	drivers/media/usb/ttusb-dec/ttusb_dec.c:380 ttusb_dec_send_command() warn: inconsistent returns '&dec->usb_mutex'.
	  Locked on  : 330
	  Unlocked on: 354,365,380

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>