net/libertas: don't recursive spin lock in if_spi_e2h()

net/libertas: don't recursive spin lock in if_spi_e2h()

|BUG: spinlock recursion on CPU#0, lbs_spi_thread/1030
| lock: dee9a1bc, .magic: dead4ead, .owner: lbs_spi_thread/1030, .owner_cpu: 0
|Call Trace:
|[deec3eb0] [c0007220] show_stack+0x4c/0x15c (unreliable)
|[deec3ef0] [c0187720] spin_bug+0x9c/0xb0
|[deec3f10] [c0187890] _raw_spin_lock+0x54/0x148
|[deec3f40] [c02d260c] _spin_lock_irqsave+0x2c/0x44
|[deec3f60] [e8671800] lbs_queue_event+0x7c/0x130 [libertas]
|[deec3f80] [e8725f04] lbs_spi_thread+0x538/0x58c [libertas_spi]
|[deec3fe0] [c004f270] kthread+0x4c/0x88
|[deec3ff0] [c000f33c] kernel_thread+0x4c/0x68

if_spi_e2h() is grabbing ->driver_lock just while calling
lbs_queue_event() which is grabbing the same lock.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>