misc: fastrpc: do not interrupt kernel calls

the DSP firmware requires some calls to be held until processing has
completed: this is to guarantee that memory continues to be
accessible.

Nevertheless, the fastrpc driver chooses not support the case were
requests need to be held for unbounded amounts of time. If such a
use-case becomes necessary, this timeout will need to be revisited.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>