ARM: stm32: Use default CONFIG_TFTP_BLOCKSIZE on DHSOM

ARM: stm32: Use default CONFIG_TFTP_BLOCKSIZE on DHSOM

The DHCOM does ship with KS8851 with 1.5 kiB packet buffer. The DHSOM
may be extended with other MAC options connected to FMC2 bus, like the
DM9000, wih similar limitations. Use default CONFIG_TFTP_BLOCKSIZE of
1468 Bytes instead of 1536 Bytes, which always avoids overflowing the
packet buffers of such limited MACs, which leads to e.g. TFTP timeouts.
This also avoids receiving a short packet fragment at the end of each
TFTP block, which led to reduced performance.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>