mtd: bcm_umi_nand: clean up error handling code

Convert error handling code to use gotos.  At the same time, this adds
calls to kfree and iounmap in a few cases where they were overlooked.

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Jiandong Zheng <jdzheng@broadcom.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>