tftp: rework the logic to validate the load address

tftp: rework the logic to validate the load address

Use the lmb_read_check() function to verify if it is safe to use a
region of memory to load data from a tftp command. The current logic
checks the amount of free memory available, starting from the 'load
address'. This call fails if the same region of memory has been used
earlier. This used to work earlier as the LMB memory map had a local
scope and was not persistent. Fix this issue by using the
lmb_read_check() call instead which only returns an error in case the
memory region has been marked for not allowing re-use.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Vaishnav Achath <vaishnav.a@ti.com>