MFD: mcp-core: fix complaints from the genirq layer

The genirq layer complains if an interrupt handler returns with
interrupts enabled.  The UCB1x00 handler does just this, because
ucb1x00_enable() calls mcp_enable(), which uses spin_lock_irq()
rather than spin_lock_irqsave().  Convert this, and the divisor
setting functions to use spin_lock_irqsave().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>