[media] smsusb: don't sleep while atomic

smscore_getbuffer() calls internally wait_event(), with can sleep.
As smsusb_onresponse() is called on interrupt context, this causes
the following warning:

	BUG: sleeping function called from invalid context at drivers/media/common/siano/smscoreapi.c:1653
	in_atomic(): 1, irqs_disabled(): 1, pid: 11084, name: systemd-udevd
	INFO: lockdep is turned off.
	irq event stamp: 0
	hardirqs last  enabled at (0): [<          (null)>]           (null)
	hardirqs last disabled at (0): [<ffffffff811480f7>] copy_process.part.7+0x10e7/0x56d0
	softirqs last  enabled at (0): [<ffffffff81148193>] copy_process.part.7+0x1183/0x56d0
	softirqs last disabled at (0): [<          (null)>]           (null)
	CPU: 2 PID: 11084 Comm: systemd-udevd Tainted: G    B   W       4.5.0-rc3+ #47
	Hardware name:                  /NUC5i7RYB, BIOS RYBDWi35.86A.0350.2015.0812.1722 08/12/2015
	 0000000000000000 ffff8803c6907a80 ffffffff81933901 ffff8802bd916000
	 ffff8802bd9165c8 ffff8803c6907aa8 ffffffff811c6af5 ffff8802bd916000
	 ffffffffa0ce9b60 0000000000000675 ffff8803c6907ae8 ffffffff811c6ce5
	Call Trace:
	 <IRQ>  [<ffffffff81933901>] dump_stack+0x85/0xc4
	 [<ffffffff811c6af5>] ___might_sleep+0x245/0x3a0
	 [<ffffffff811c6ce5>] __might_sleep+0x95/0x1a0
	 [<ffffffffa0ce020a>] ? list_add_locked+0xca/0x140 [smsmdtv]
	 [<ffffffffa0ce3b8d>] smscore_getbuffer+0x7d/0x120 [smsmdtv]
	 [<ffffffff8123819d>] ? trace_hardirqs_off+0xd/0x10
	 [<ffffffffa0ce3b10>] ? smscore_sendrequest_and_wait.isra.5+0x120/0x120 [smsmdtv]
	 [<ffffffffa0ce020a>] ? list_add_locked+0xca/0x140 [smsmdtv]
	 [<ffffffffa0ce13ca>] ? smscore_putbuffer+0x3a/0x40 [smsmdtv]
	 [<ffffffffa0d107bc>] smsusb_submit_urb+0x2ec/0x4f0 [smsusb]
	 [<ffffffffa0d10e36>] smsusb_onresponse+0x476/0x720 [smsusb]

Let's add a work queue to handle the bottom half, preventing this
problem.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>