staging: greybus: firmware: Convert sscanf calls to strtoul

Also convert the fw_update_type and fw_timeout variables to
unsigned and update the printf specifier to %u.

The FW_MGMT_IOC_SET_TIMEOUT_MS ioctl takes an unsigned int
and checkpatch was complaining about not checking the sscanf
return values.

Signed-off-by: Michael Sartain <mikesart@fastmail.com>
Acked-by: Viresh Kumar <viresh.kumar at linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>