uvesafb: print error message when task timeout occurs

The driver waits for response from user space for a pending
task until a timeout (UVESAFB_TIMEOUT) occurs. But the
existing error message in later steps is a little obscure.

This patch throws out an error message when timeout happens.

Signed-off-by: Jianxun Zhang <jianxun.zhang@linux.intel.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>