soc: qcom: spcom: fix error code check to resend packet to the peer

spcom driver implements RPMSG client, using glink as transport to
communicate with Secure Processor. SPU glink peer do not supports rx
intent requests, so spcom driver always uses rpmsg_trysend() api to send
packet.

This change fixes the value to repeat send attempt to be -EBUSY and
-EAGAIN.
Also, spcom_device_write() fixed to propagate error code to userspace.

Change-Id: Iaac9bb862a6126fe5b18a22b2c7ee50247f71438
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>