ptp: Simplify ptp_read()

The mixture of gotos and direct return codes is inconsistent and just makes
the code harder to read. Let it consistently return error codes directly and
tidy the code flow up accordingly.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20250625115133.486953538@linutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>