power: qpnp-linear-charger: avoid mutex_lock in atomic context

Spin_lock_irqsave function called in get_prop_capacity() introduces
an atomic context. A mutex_lock is called in the context and this
results a warning message. Use mutex instead of spin_lock to avoid
this condition.

Change-Id: I8098f74bb6888f4cb514199c5636d3fc6cede91d
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>