oem-ibm: drop redundant c_str() in CertHandler I/O calls

Remove redundant c_str() calls when passing file paths to
transferFileData() and readFile() in CertHandler.

The stlcstrParam warning reports unnecessary conversions of
std::string objects to C-style strings when the called functions
already accept std::string-compatible parameters. Removing the
explicit c_str() calls improves readability and follows modern
C++ practices.

Modified two call sites in
oem/ibm/libpldmresponder/file_io_type_cert.cpp:

- transferFileData(filePath.c_str(), ...)
  -> transferFileData(filePath, ...)
- readFile(filePath.c_str(), ...)
  -> readFile(filePath, ...)

No functional change intended.

Change-Id: I04236f19b79fe45d8a5d8110dfdea06d876b33d0
Signed-off-by: Meghana Vangapandu <meghanav@ami.com>