net: tftp: Fix tftp_prev_block counter update

Fixes missing update to tftp_prev_block counter before increase
tftp_cur_block counter when do the tftpput operation.

tftp_prev_block counter is used in update_block_number() function to
check whether block number (sequence number) is rollover. This bug
cause the tftpput command fail to upload a large file when block
number is greater than 16-bit (0xFFFF).

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>