ARM: stm32: Use CONFIG_TFTP_TSIZE on DHSOM

ARM: stm32: Use CONFIG_TFTP_TSIZE on DHSOM

Long TFTP transfers lead to a wall of # characters on UART, which in
the end may slow down the transfer itself. Use CONFIG_TFTP_TSIZE to
print progress in fewer # characters.

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>