usb: typec: ucsi: Move unregister out of atomic section

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

usb: typec: ucsi: Move unregister out of atomic section

Commit '9329933699b3 ("soc: qcom: pmic_glink: Make client-lock

non-sleeping")' moved the pmic_glink client list under a spinlock, as it

is accessed by the rpmsg/glink callback, which in turn is invoked from

IRQ context.

This means that ucsi_unregister() is now called from atomic context,

which isn't feasible as it's expecting a sleepable context. An effort is

under way to get GLINK to invoke its callbacks in a sleepable context,

but until then lets schedule the unregistration.

A side effect of this is that ucsi_unregister() can now happen

after the remote processor, and thereby the communication link with it, is

gone. pmic_glink_send() is amended with a check to avoid the resulting NULL

pointer dereference.

This does however result in the user being informed about this error by

the following entry in the kernel log:

ucsi_glink.pmic_glink_ucsi pmic_glink.ucsi.0: failed to send UCSI write request: -5