mfd: ezx-pcap: Replace mutex_lock with spin_lock

As mutex_lock might sleep. Function pcap_adc_irq is an interrupt handler.
The use of mutex_lock in pcap_adc_irq may cause sleep in IRQ context.
Replace mutex_lock with spin_lock to avoid this.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>