[SCSI] atp870u: reduce huge stack usage

The atp870u driver is the largest stack eater reported by checkstack
(on x86_864, allmodconfig).  This converts the offending function
to kmalloc+kfree struct atp_unit instead of allocating it on the stack.
Was:
0x0000164c atp870u_probe [atp870u]:			3176
Now:
0x0000164c atp870u_probe [atp870u]:			408

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>