cfi_flash: make the command u32 only

This got changed by commit 93c56f21
[cfi_flash: support of long cmd in U-boot.]

Long is the wrong type because it will behave differently on 64bit
machines in a way that is probably not expected. u32 should be
enough.

Cc: Alexey Korolev <akorolev@infradead.org>
Cc: Vasiliy Leonenko <vasiliy.leonenko@mail.ru>
Signed-off-by: Sebastian Siewior <bigeasy@linutronix.de>