fsi: core: Fix return of error values on failures

fsi: core: Fix return of error values on failures

Currently the cfam_read and cfam_write functions return the provided
number of bytes given in the count parameter and not the error return
code in variable rc, hence all failures of read/writes are being
silently ignored. Fix this by returning the error code in rc.

Addresses-Coverity: ("Unused value")
Fixes: d1dcd6782576 ("fsi: Add cfam char devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Jeremy Kerr <jk@ozlabs.org>
Link: https://lore.kernel.org/r/20210603122812.83587-1-colin.king@canonical.com
Signed-off-by: Joel Stanley <joel@jms.id.au>