drivers:misc: ti-st: protect registrations

Concurrent access to UART2/combo-interface by multiple protocol drivers such
as BT, FM and GPS caused issues during firmware download failure cases or
cases when the firmware download took longer than usual.

This was because of un-safe access to protos_registered & st_states.
Protecting this will also make the registration complete callback un-safe for
sleep.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>