lmb: optimise the lmb allocation functions

lmb: optimise the lmb allocation functions

The actual logic to allocate a region of memory is in the
_lmb_alloc_base() function. The lmb_alloc() API function calls
lmb_alloc_base(), which then calls _lmb_alloc_base() to do the
allocation. Instead, call the _lmb_alloc_base() directly from both the
allocation API's, and move the error message to the _lmb_alloc_base().

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>