net: mctp: fix don't require received header reserved bits to be zero

From the MCTP Base specification (DSP0236 v1.2.1), the first byte of
the MCTP header contains a 4 bit reserved field, and 4 bit version.

On our current receive path, we require those 4 reserved bits to be
zero, but the 9500-8i card is non-conformant, and may set these
reserved bits.

DSP0236 states that the reserved bits must be written as zero, and
ignored when read. While the device might not conform to the former,
we should accept these message to conform to the latter.

Relax our check on the MCTP version byte to allow non-zero bits in the
reserved field.

Fixes: 889b7da23abf ("mctp: Add initial routing framework")
Signed-off-by: Yuan Zhaoming <yuanzm2@lenovo.com>
Cc: stable@vger.kernel.org
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20260417141340.5306-1-yuanzhaoming901030@126.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>