Revert "drivers: modem_if: use memcpy in dpram_download for header"

-limiting this to only P4LTE

This reverts commit 0dc15f580424db4d11928e0fcc1e72e081cba444.
drivers: modem_if: p4lte: use memcpy in dpram_download for header

-using inline _memcpy for this triggers an internal compiler error in
 GCC 4.6.