staging: westbridge: improve error path

kmalloc() may fail, check for it.
Allocated memory is not freed.
Use IS_ERR() instead of strict checking.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Cc: David Cross <david.cross@cypress.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>