Flesh the duplicated functionality to a helper.
Use const char * + sscanf to keep the compiler happy.

strtol discards the const qualifier of the endptr string. One could
workaround it by using a local variable. Yet, sscanf seems like a
shorter way to achieve the goal.
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>