mctp i2c: Fix hard head TX bounds length check

We should be testing the length before fitting into the u8 byte_count.
This is just a sanity check, the MCTP stack should have limited to MTU
which is checked, and we check consistency later in mctp_i2c_xmit().

Found by Smatch
mctp_i2c_header_create() warn: impossible condition
    '(hdr->byte_count > 255) => (0-255 > 255)'

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>