libflash/mbox-flash: Fix unintentional integer overflow (CID 142226)
libflash/mbox-flash: Fix unintentional integer overflow (CID 142226)

Fixes: CID 142226 (#1 of 1):
overflow_before_widen: Potentially overflowing expression
`1 << mbox_flash->shift` with type int (32 bits, signed) is evaluated using
32-bit arithmetic, and then used in a context that expects an expression
of type uint64_t (64 bits, unsigned).

Fixes: CID 142226
Signed-off-by: Cyril Bur <6ce38dade8c4b52fd55c193fae70fd1d1275c3c3@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,shenki/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot