scsi: pm80xx: Set phy->enable_completion only when we wait for it

In the Linux kernel, the following vulnerability has been resolved:

scsi: pm80xx: Set phy->enable_completion only when we wait for it

pm8001_phy_control() populates the enable_completion pointer with a stack

address, sends a PHY_LINK_RESET / PHY_HARD_RESET, waits 300 ms, and

returns. The problem arises when a phy control response comes late. After

300 ms the pm8001_phy_control() function returns and the passed

enable_completion stack address is no longer valid. Late phy control

response invokes complete() on a dangling enable_completion pointer which

leads to a kernel crash.