Staging: mt29f_spinand: Remove redundant cast on 'retval'

Remove the unneeded cast to 'int' since 'retval' is originally declared as
'int'.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>