mfd: twl4030-irq: Fix style warnings relating to pr_warn() and sizeof()

This is part of an effort to clean-up the MFD subsystem.

WARNING: Prefer pr_warn(... to pr_warning(...
+               pr_warning("twl4030: I2C error %d reading PIH ISR\n", ret);

WARNING: sizeof buf should be sizeof(buf)
+       memset(buf, 0xff, sizeof buf);

WARNING: sizeof *agent should be sizeof(*agent)
+       agent = kzalloc(sizeof *agent, GFP_KERNEL);

Signed-off-by: Lee Jones <lee.jones@linaro.org>