jsm: fixing error if the driver fails to load

Currently if the driver fails to register on port, the kernel
crashes with the following stack:

cpu 0x1: Vector: 300 (Data Access) at [c0000000e0303090]
    pc: c00000000039aa74: .__mutex_lock_slowpath+0x44/0x10c
    lr: c00000000039aa58: .__mutex_lock_slowpath+0x28/0x10c
[c0000000e03033c0] c00000000026b074 .uart_remove_one_port+0xbc/0x16c
[c0000000e0303460] d0000000000e0554 .jsm_remove_uart_port+0x8c/0x10c [jsm]
[c0000000e03034f0] d0000000000dc034 .jsm_remove_one+0x34/0x108 [jsm]
[c0000000e0303590] c0000000001f4aa0 .pci_device_remove+0x48/0x74
...

This patch just fixes the code flow to abort the load when an
error is detected.

Signed-off-by: Breno Leitao <leitao@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>