drivers/eeprom/eeprom_emulator: fix mutex usage with rambuf

In the eeprom read operation, when rambuf was available
mutex was not unlocked after the read. Consequence of that is
that device was blocked after that read for incoming operations.

This commit fixes the issue

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>