msm: camera: core: Avoid spinlock recusion

The context's spinlock is shared between ioctl's and
tasklet. There is a possibility that while holding a
spinlock a tasklet gets scheduled on the same core.
This can cause spinlock recursion as the variants of
spin_lock used are different.
To fix this use spin_lock_bh instead of spin_lock.

Change-Id: I882e40d6c5da349da8b828cd6b2806828e1ea0a6
Signed-off-by: Abhilash Kumar <krabhi@codeaurora.org>