ipc: static_vrings: Fix timeout management

The ipc_service_get_tx_buffer() has a timeout parameter that can be used
to wait a certain amount of time for a TX buffer to be available.

Unfortunately, for the static vrings backend, an asymmetry
between remote and host exists that makes the usage of this parameter
confusing when the user requests a buffer when no buffers are available
at that time.

When the remote endpoints requests a TX buffer specifying a certain
size and there are no TX buffers available, the function ignores the
parameter and ipc_service_get_tx_buffer() immediately returns -ENOMEM.

The same case on the host endpoint works correctly only when the
specified timeout is <= 15 seconds. All timeouts > 15 seconds simply
returns -EIO after 15 seconds.

This patch is reworking the timeout management trying to behave
correctly in all the cases.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>