soc: qcom: mdt_loader: Rename mdt_phdr_valid()

The function checks if a program header refers to a PT_LOAD segment,
that isn't a hash segment (which should be PT_LOAD in the first place),
andwith non-zero size. That's not the definition of "valid", but rather
if it's "loadable".

Rename the function to reflect what it does.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250610-mdt-loader-validation-and-fixes-v2-2-f7073e9ab899@oss.qualcomm.com


Signed-off-by: Bjorn Andersson <andersson@kernel.org>