misc: make format_hexdump length u32

misc: make format_hexdump length u32

format_hexdump currently requires the length parameter to be uword
(64-bits) hence all callers must make sure to cast the length to uword.
Use u32 instead to benefit from C automatic integer promotion: any
length smaller or equal to u32 will be promoted to int fitting in u32).
Only callers using a length of u64 needs to downcast.
It also makes it similar to other variants.

Type: fix

Change-Id: I09b52fdde3970cec0be4150a29126ff63106c75b
Signed-off-by: Benoît Ganne <bganne@cisco.com>