allow multiple deices on SPI bus

[dariosalvi78]

PROBLEM:
with 2 devices on the SPI bus, the SD card library fails when calling the begin() function
the problem is that the library is not prepared to work with other SPI devices, it assumes you only have the SD card basically

SOLUTION:
in the file named SD.cpp, in function begin() (line 345) add this instruction before the return statement:

if(root.isOpen()) root.close();

that should do it.

[sandeepmistry]

Hi @dariosalvi78, could you please submit a pull request for this suggestion.