Increase maximum number of session of the internal TFTP server.

Grub fails to boot from internal TFTP server when loading more than
3 initrd files.

Grub first opens a session to the TFTP server for every initrd file and
retrieves only the file size for all.
Then it wants to download the content using the old sessions which are
already expired.

Increasing the maximum number of session of the internal TFTP
server avoids this issue.

The error message reads as following:
error: timeout reading
`/boot/ISO.ROOT/BOOTMGR'.

Press any key to continue...

Signed-off-by: Bernhard Übelacker <bernhardu@vr-web.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>