i2c-omap: Fix NULL pointer dereferencing

This patch fixes bug #9581 reported by Marcio Buss. If kzalloc fails,
omap_i2c_write_reg() tries to reset an unallocated I2C controller.

Cc: Marcio Buss <marciobuss@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>