target-mips: fix memory leak

Coveristy reports that variable prom_buf/params_buf going
out of scope leaks the storage it points to.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
